Add GraphQL mutation to create a new release

What does this MR do?

Adds a GraphQL mutation to create a new release.

This is the GraphQL equivalent of the POST api/v4/projects/:id/releases API endpoint. It follows the same permission model (only Developers or higher are allowed to create releases.)

Example query/response

Query:

mutation ($input: ReleaseCreateInput!) {
  releaseCreate(input: $input) {
    release {
      tagName
      name
      description
      createdAt
      releasedAt
      milestones {
        nodes {
          title
        }
      }
      assets {
        links {
          nodes {
            name
            url
            linkType
            external
            directAssetUrl
          }
        }
      }
    }
    errors
  }
}

Variables:

{
  "input": {
    "projectPath": "root/release-test",
    "tagName": "v6.0.68",
    "ref": "master",
    "name": "GraphQL Release",
    "description": "A release created through our GraphQL endpoint :rocket:",
    "milestones": ["12.3", "12.4"],
    "releasedAt": "2018-12-10",
    "assets": {
      "links": [{
        "name": "A link with a permanent path",
        "url": "https://example.com/link",
        "filepath": "/my/link",
        "linkType": "OTHER"
      }]
    }
  }
}

When the user has at least Developer permissions

Response:

{
  "data": {
    "releaseCreate": {
      "release": {
        "tagName": "v6.0.68",
        "name": "GraphQL Release",
        "description": "A release created through our GraphQL endpoint :rocket:",
        "createdAt": "2020-10-30T17:47:49Z",
        "releasedAt": "2018-12-10T05:00:00Z",
        "milestones": {
          "nodes": [
            {
              "title": "12.4"
            },
            {
              "title": "12.3"
            }
          ]
        },
        "assets": {
          "links": {
            "nodes": [
              {
                "name": "A link with a permanent path",
                "url": "https://example.com/link",
                "linkType": "OTHER",
                "external": true,
                "directAssetUrl": "http://0.0.0.0:3000/root/release-test/-/releases/v6.0.68/my/link"
              }
            ]
          }
        }
      },
      "errors": []
    }
  }
}

When a validation error occurs

Response:

{
  "data": {
    "releaseCreate": {
      "release": null,
      "errors": [
        "Release already exists"
      ]
    }
  }
}

When the user has at most Reporter permissions

Response:

{
  "data": {
    "releaseCreate": null
  },
  "errors": [
    {
      "message": "The resource that you are attempting to access does not exist or you don't have permission to perform this action",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "releaseCreate"
      ]
    }
  ]
}

Related to #243727 (closed)