Provide Release JSON schema for Evidence collection

What does this MR do?

Releted MR: !17335 (merged)

The goal of this MR is to provide a first version of the JSON schema that will be used in RSpec. It'll be used to check the format of the JSON object created when creating Evidence object.

Here's the JSON sample that's used as a reference for this MR:

{
  "release": {
    "id": 5,
    "tag_name": "v4.0",
    "name": "New release",
    "description": "This is a new release",
    "created_at": "2019-06-28 13:23:40 UTC",
    "project": {
        "id": 45,
        "name": "Project name",
        "description": "This is a detailed project description",
        "created_at": "2017-02-22 11:25:12 UTC"
    }
    "milestones": [
      {
        "id": 11,
        "title": "v4.0-rc1",
        "description": "This is the description of milestone #11",
        "state": "closed",
        "iid": 1,
        "created_at": "2019-04-17 15:45:12 UTC",
        "due_date": "2019-05-12 12:00:00 UTC",
        "issues": [
          {
            "id": 82,
            "title": "The top-right popup is broken",
            "description": "This is a description with more details for this issue",
            "state": "closed",
            "iid": 1,
            "confidential": true,
            "created_at": "2019-04-17 15:45:12 UTC",
            "due_date": "2019-05-10 12:00:00 UTC"
          },
          {
            "id": 89,
            "title": "The title of this page is misleading",
            "description": "This is a description with more details for this issue",
            "state": "closed",
            "iid": 2,
            "confidential": false,
            "created_at": "2019-04-17 15:45:12 UTC",
            "due_date": "nil"
          }
        ]
      },
      {
        "id": 12,
        "title": "v4.0-rc2",
        "description": "This is the description of milestone #12",
        "state": "closed",
        "iid": 2,
        "due_date": "2019-05-30 18:30:00 UTC",
        "created_at": "2019-04-17 15:45:12 UTC",
        "issues": []
      }
    ]
  }
}

cc @ogolowinski

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Related to #26019 (closed)