Add MergeRequest.reviewers

What does this MR do?

This MR exposes reviewers of merge requests in GraphQL.

Reviewers are returned if and only if the merge request allows reviewers, otherwise the field resolves to null.

Tests are added for this field, and it is guaranteed to be N+1 safe, by adding the appropriate preloads to the merge request loader.

GraphQL Queries

Newly supported queries:

query {
  currentUser {
    authoredMergeRequests {
      nodes {
        reviewers {
          nodes { username }
        }
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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