Add forkDetails GraphQL field to ProjectType

What does this MR do and why?

Add forkDetails GraphQL field to ProjectType . The service has been introduced in !103814 (merged), but the feature has been disabled by default because the calculation slows down the page (even though it's cached). This MR introduces a GraphQL endpoint in order to load these counts async.

{
  project(fullPath: "gitlab-org/gitlab-shell") {
    forkDetails(ref: "main"){
      ahead
      behind
    }
  }
}