Expose merge request entity for pipelines

What does this MR do?

This MR exposed the following properties of a merge request for pipeline.

expose :merge_request do
  expose :iid
  expose :merge_request_path, as: :path
  expose :title
  expose :source_branch
  expose :source_branch_path
  expose :target_branch
  expose :target_branch_path
end

expose :flags do
  ...
  expose :detached?, as: :detached
end

These values are used for both attached and detached pipeline. The UX mock is displayed in https://gitlab.com/gitlab-org/gitlab-ee/issues/7380.

Backend just needs to expose there values in PipelineEntity only. This is because all the related endpoints go through this entity class.

Sample JSON reponse

{
    "id": 995,
    "name": "job",
    "started": "2019-02-28T07:01:08.840Z",
    "archived": false,
    "build_path": "/root/detached-merge-request-pipelines/-/jobs/995",
    "retry_path": "/root/detached-merge-request-pipelines/-/jobs/995/retry",
    "playable": false,
    "scheduled": false,
    "created_at": "2019-02-28T07:01:07.621Z",
    "updated_at": "2019-02-28T07:01:09.709Z",
    "status": {
        "icon": "status_success",
        "text": "passed",
        "label": "passed",
        "group": "success",
        "tooltip": "passed",
        "has_details": true,
        "details_path": "/root/detached-merge-request-pipelines/-/jobs/995",
        "illustration": {
            "image": "/assets/illustrations/skipped-job_empty-8b877955fbf175e42ae65b6cb95346e15282c6fc5b682756c329af3a0055225e.svg",
            "size": "svg-430",
            "title": "This job does not have a trace."
        },
        "favicon": "/assets/ci_favicons/favicon_status_success-8451333011eee8ce9f2ab25dc487fe24a8758c694827a582f17f42b0a90446a2.png",
        "action": {
            "icon": "retry",
            "title": "Retry",
            "path": "/root/detached-merge-request-pipelines/-/jobs/995/retry",
            "method": "post",
            "button_title": "Retry this job"
        }
    },
    "coverage": null,
    "erased_at": null,
    "duration": 0.861057,
    "tags": [],
    "has_trace": true,
    "stage": "test",
    "stuck": false,
    "user": {
        "id": 1,
        "name": "Administrator",
        "username": "root",
        "state": "active",
        "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
        "web_url": "http://localhost:8181/root",
        "status_tooltip_html": null,
        "path": "/root"
    },
    "runner": {
        "id": 1,
        "description": "shinya-Galago-Pro",
        "edit_path": "/root/detached-merge-request-pipelines/runners/1/edit"
    },
    "pipeline": {
        "id": 49,
        "user": {
            "id": 1,
            "name": "Administrator",
            "username": "root",
            "state": "active",
            "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
            "web_url": "http://localhost:8181/root",
            "status_tooltip_html": null,
            "path": "/root"
        },
        "active": false,
        "coverage": null,
        "source": "merge_request",
        "created_at": "2019-02-28T05:34:36.364Z",
        "updated_at": "2019-02-28T07:01:10.076Z",
        "path": "/root/detached-merge-request-pipelines/pipelines/49",
        "flags": {
            "latest": true,
            "stuck": false,
            "auto_devops": false,
            "merge_request": true,
            "detached": true,
            "yaml_errors": false,
            "retryable": false,
            "cancelable": false,
            "failure_reason": false
        },
        "details": {
            "status": {
                "icon": "status_success",
                "text": "passed",
                "label": "passed",
                "group": "success",
                "tooltip": "passed",
                "has_details": true,
                "details_path": "/root/detached-merge-request-pipelines/pipelines/49",
                "illustration": null,
                "favicon": "/assets/ci_favicons/favicon_status_success-8451333011eee8ce9f2ab25dc487fe24a8758c694827a582f17f42b0a90446a2.png"
            },
            "duration": 0,
            "finished_at": "2019-02-28T07:01:10.061Z"
        },
        "merge_request": {
            "iid": 1,
            "path": "/root/detached-merge-request-pipelines/merge_requests/1",
            "title": "Update README.md",
            "source_branch": "feature-1",
            "source_branch_path": "/root/detached-merge-request-pipelines/branches/feature-1",
            "target_branch": "master",
            "target_branch_path": "/root/detached-merge-request-pipelines/branches/master"
        },
        "ref": {
            "name": "refs/merge-requests/1/head",
            "path": "/root/detached-merge-request-pipelines/commits/refs/merge-requests/1/head",
            "tag": false,
            "branch": false,
            "merge_request": true
        },
        "commit": {
            "id": "4315968cb4a3137eab5c517892d06ad379b6010b",
            "short_id": "4315968c",
            "created_at": "2019-02-28T05:34:30.000Z",
            "parent_ids": [
                "941c1a011d85ab52ee9c04bfc9d474f8c8d0e3fd"
            ],
            "title": "Update README.md",
            "message": "Update README.md",
            "author_name": "Administrator",
            "author_email": "admin@example.com",
            "authored_date": "2019-02-28T05:34:30.000Z",
            "committer_name": "Administrator",
            "committer_email": "admin@example.com",
            "committed_date": "2019-02-28T05:34:30.000Z",
            "author": {
                "id": 1,
                "name": "Administrator",
                "username": "root",
                "state": "active",
                "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
                "web_url": "http://localhost:8181/root",
                "status_tooltip_html": null,
                "path": "/root"
            },
            "author_gravatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
            "commit_url": "http://localhost:8181/root/detached-merge-request-pipelines/commit/4315968cb4a3137eab5c517892d06ad379b6010b",
            "commit_path": "/root/detached-merge-request-pipelines/commit/4315968cb4a3137eab5c517892d06ad379b6010b"
        }
    },
    "metadata": {
        "timeout_human_readable": "1h",
        "timeout_source": "project"
    },
    "artifact": {},
    "erase_path": "/root/detached-merge-request-pipelines/-/jobs/995/erase",
    "raw_path": "/root/detached-merge-request-pipelines/-/jobs/995/raw",
    "runners": {
        "online": false,
        "available": true,
        "settings_path": "/root/detached-merge-request-pipelines/runners"
    }
}

What are the relevant issue numbers?

Related gitlab-ee#7380 #58454 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer