Add milestone stats to GraphQL endpoint

What does this MR do?

Adds milestones statistics to the GraphQL endpoint (exposed as a stats field).

This data is not available through the Milestones API, but is available when getting milestone data through the Releases API. Because of this, this data is necessary to make the release GraphQL data complete.

In the Releases API, this data is exposed under a issue_stats field. This MR exposes these fields under a more generic stats field instead, because merge request statistics will eventually be added to this type.

Testing

This MR also augments some of the existing milestone GraphQL tests to allow the new stats field to be tested. The tests are still by no means comprehensive, but they are a little more thorough than before.

Feature flag

The new field being added as part of this MR - stats - is hidden behind a graphql_milestone_stats feature flag, which is enabled by default.

If this feature flag is disabled, the stats field will still be query-able, but will always return null.

Example query/response

Query:

{
  group(fullPath: "my-example-group") {
    milestones {
      nodes {
        title
        stats {
          closedIssuesCount
          totalIssuesCount
        }
      }
    }
  }
}

Response:

{
  "data": {
    "group": {
      "milestones": {
        "nodes": [
          {
            "title": "My Example Milestone",
            "stats": {
              "closedIssuesCount": 1,
              "totalIssuesCount": 2
            }
          }
        ]
      }
    }
  }
}

Response when the graphql_milestone_stats feature flag is disabled:

{
  "data": {
    "group": {
      "milestones": {
        "nodes": [
          {
            "title": "My Example Milestone",
            "stats": null
          }
        ]
      }
    }
  }
}

Related to #208702 (closed)