Expose 'canMerge' permission for merge request assignees on GraphQL API

Summary

Currently, when fetching a list of assignees of the given merge request via GraphQL API, we don't have a chance to define if an assignee has permission to merge a merge request. To have parity with REST API, we need to expose this permission in the GraphQL API as well.

Example query

query {
  project(fullPath: "gitlab-org/gitlab") {
    mergeRequest(iid: "55944") {
      author { ...uf }
      reviewers { nodes { ...uf mergeRequestInteraction { canMerge } }}
      assignees { nodes { ...uf mergeRequestInteraction { canMerge } }}
    }
  }
}


fragment uf on User {
  username
}

The same field should be exposed in MergeRequestSetAssigneesPayload type.