Added reviewer_wildcard_id filtering to GraphQL API

What does this MR do and why?

Adds a new argument to filter merge requests by reviewer wildcard

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

• Open http://gdk.test:3000/dashboard/merge_requests
• Run a query like:

query {
  currentUser {
    assignedMergeRequests(state:opened, reviewerWildcardId:NONE) {
      nodes {
        id
        webUrl
      }
    }
  }
}

It should return all merge requests your user is an assignee of that has no reviewers