Dependency List private API endpoint

This issue follows https://gitlab.com/gitlab-org/gitlab-ee/issues/11060 and its aim is to have a working endpoint that returns real data about dependencies used in the project.

List of issues

• Parse "dependency_files" field of gl-dependency-scanning-report.json report.
• Prepare that information to be consumed by frontend without additional alteration. All changes should be done on backend side.
• Alter current endpoint response, remove all mock data.
• Paginate the returned dependencies if the page parameter is present in the request; otherwise, return the full list
• Add handling for known edge cases.
• Add usage ping for this endpoint. https://gitlab.com/gitlab-org/gitlab-ee/issues/10075#note_167662681 for more info.

Request and response format

GET /:namespace_id/:project_id/security/dependencies.json

[
  { 
   dependencies: [
     {  
        "name": "rails",
        "packager": "Ruby (Bundler)",
        "location": {
          "blob_path": "/group-name/project-name/blob/deb6f84e91fe4d21daa6b5558c517254ea2668a3/Gemfile.lock"
          "path": "Gemfile.lock",
        },
        "version": "5.2.3"
     },
     ...
   ],
   report: {
     status: "some_status",
     job_path: "some_ci_job_path"
   }
}

What changed since 11.11?

• type => packager
• location.blob_path provides only information about the file path, not the line number of dependency because the current scanner isn't able to provide this information.
• dependency.requirements is removed this the scanner can't provide this information.
• we have in the same time info about dependencies and report status.

Edge case statuses

• job_not_set_up Design
• job_failed || no_dependencies Design
• no_dependency_files Design

Note: How backend will know about this statuses TBA

Note: If everything is OK, status ok is returned.

/cc @markrian @fcatteau