Expose upcoming deployment in environment.json

What does this MR do?

This MR exposes an upcoming deployment information in environment.json internal API.

Here is an example payload:

"upcoming_deployment": {
          "id": 141,
          "iid": 21,
          "sha": "eb60d65ff4d57ee7c38e0cb29ac05e716a93af2e",
          "ref": {
            "name": "master",
            "ref_path": "/root/auto-rollback-test-2/-/tree/master"
          },
          "status": "created",
          "created_at": "2020-11-24T08:38:05.313Z",
          "deployed_at": null,
          "tag": false,
          "last?": 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://local.gitlab.test:8181/root",
            "status_tooltip_html": null,
            "path": "/root"
          },
          "deployable": {
            "id": 979,
            "name": "deploy",
            "started": false,
            "archived": false,
            "build_path": "/root/auto-rollback-test-2/-/jobs/979",
            "cancel_path": "/root/auto-rollback-test-2/-/jobs/979/cancel?continue%5Bto%5D=%2Froot%2Fauto-rollback-test-2%2F-%2Fjobs%2F979",
            "playable": false,
            "scheduled": false,
            "created_at": "2020-11-24T08:38:05.283Z",
            "updated_at": "2020-11-24T08:38:05.378Z",
            "status": {
              "icon": "status_pending",
              "text": "pending",
              "label": "pending",
              "group": "pending",
              "tooltip": "pending",
              "has_details": true,
              "details_path": "/root/auto-rollback-test-2/-/jobs/979",
              "illustration": {
                "image": "/assets/illustrations/pending_job_empty-3eb317d31c83d253ed6d54826f6f4e5315f25a51b1ce28481806a52b5d5ed20b.svg",
                "size": "svg-430",
                "title": "This job has not started yet",
                "content": "This job is in pending state and is waiting to be picked by a runner"
              },
              "favicon": "/assets/ci_favicons/favicon_status_pending-5bdf338420e5221ca24353b6bff1c9367189588750632e9a871b7af09ff6a2ae.png",
              "action": {
                "icon": "cancel",
                "title": "Cancel",
                "path": "/root/auto-rollback-test-2/-/jobs/979/cancel",
                "method": "post",
                "button_title": "Cancel this job"
              }
            }
          },
          "commit": {
            "id": "eb60d65ff4d57ee7c38e0cb29ac05e716a93af2e",
            "short_id": "eb60d65f",
            "created_at": "2020-11-23T07:25:24.000+00:00",
            "parent_ids": ["e153ae118908fce025de3783f7e15aeae1108bb3"],
            "title": "Update README.md",
            "message": "Update README.md",
            "author_name": "Administrator",
            "author_email": "admin@example.com",
            "authored_date": "2020-11-23T07:25:24.000+00:00",
            "committer_name": "Administrator",
            "committer_email": "admin@example.com",
            "committed_date": "2020-11-23T07:25:24.000+00:00",
            "web_url": "http://local.gitlab.test:8181/root/auto-rollback-test-2/-/commit/eb60d65ff4d57ee7c38e0cb29ac05e716a93af2e",
            "author": {
              "id": 1,
              "name": "Administrator",
              "username": "root",
              "state": "active",
              "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
              "web_url": "http://local.gitlab.test:8181/root",
              "status_tooltip_html": null,
              "path": "/root"
            },
            "author_gravatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon",
            "commit_url": "http://local.gitlab.test:8181/root/auto-rollback-test-2/-/commit/eb60d65ff4d57ee7c38e0cb29ac05e716a93af2e",
            "commit_path": "/root/auto-rollback-test-2/-/commit/eb60d65ff4d57ee7c38e0cb29ac05e716a93af2e"
          }
        },

Related #232494 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team