WIP: POC of Release data in GraphQL

What does this MR do?

Adds all data currently available in our Release API to our GraphQL endpoint.

For now, this MR is just a POC to get feedback and direction.

Tests

This MR does not yet contain tests.

API shape

The shape of the GraphQL response mimics the shape of our existing Release REST API.

Permissions

Permissions line up with the existing project-level Release permissions.

In particular, if the user only has Guest permissions, sources will return [] and commit will return null. (See example response below.)

Feature flag

This new data is hidden behind a graphql_release_data feature flag, which is disabled by default.

Example query/response

Query:

{
  project(fullPath: "root/release-test") {
    releases(first: 1) {
      nodes {
        name
        tagName
        tagPath
        description
        descriptionHtml
        evidenceSha
        createdAt
        updatedAt
        releasedAt
        assets {
          assetsCount
          sources {
            format
            url
          }
          links {
            id
            name
            url
            external
          }
          evidenceFilePath
        }
        author {
          name
          username
        }
        commit {
          sha
          webUrl
        }
        milestones {
          nodes {
            title
          }
        }
      }
    }
  }
}

Response:

{
  "data": {
    "project": {
      "releases": {
        "nodes": [
          {
            "name": "Version 16.5",
            "tagName": "v16.5",
            "tagPath": "/root/release-test/-/tags/v16.5",
            "description": "This is a small update to version sixteen.\n\nAnd this is an edit!\n\nAnd this is an edit through the API!",
            "descriptionHtml": "<p data-sourcepos=\"1:1-1:42\" dir=\"auto\">This is a small update to version sixteen.</p>&#x000A;<p data-sourcepos=\"3:1-3:20\" dir=\"auto\">And this is an edit!</p>&#x000A;<p data-sourcepos=\"5:1-5:36\" dir=\"auto\">And this is an edit through the API!</p>",
            "evidenceSha": "08f63709250846dc29b9108bcbc90cd3dcecbc9ce02d",
            "createdAt": "2019-11-21T18:45:49Z",
            "updatedAt": "2020-03-11T15:54:44Z",
            "releasedAt": "2020-06-21T18:45:49Z",
            "assets": {
              "assetsCount": 5,
              "sources": [
                {
                  "format": "zip",
                  "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.zip"
                },
                {
                  "format": "tar.gz",
                  "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar.gz"
                },
                {
                  "format": "tar.bz2",
                  "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar.bz2"
                },
                {
                  "format": "tar",
                  "url": "http://0.0.0.0:3001/root/release-test/-/archive/v16.5/release-test-v16.5.tar"
                }
              ],
              "links": [
                {
                  "id": "gid://gitlab/Releases::Link/7",
                  "name": "Blog post",
                  "url": "https://google.com",
                  "external": true
                }
              ],
              "evidenceFilePath": "http://0.0.0.0:3001/root/release-test/-/releases/v16.5/evidence.json"
            },
            "author": {
              "name": "Nathan Friend",
              "username": "root"
            },
            "commit": {
              "sha": "87fdf474c9e453a8e9ece1353fe7ff915b1e9ccd",
              "webUrl": "http://0.0.0.0:3001/root/release-test/-/commit/87fdf474c9e453a8e9ece1353fe7ff915b1e9ccd"
            },
            "milestones": {
              "nodes": [
                {
                  "title": "13.20"
                }
              ]
            }
          }
        ]
      }
    }
  }
}

Related to: #208702 (closed), #208724 (closed)