Change type of merge request assignees in GraphQL

What does this MR do?

This MR addresses #299881 (closed) by adding the merge request interaction field to MergeRequest.assignees.

This MR converts the User type from a concrete type to an interface, with UserCore as the primary concrete implementation.

The reason for this is to enable more specific kinds of user with new fields (see MergeRequestAssignee) but without breaking the use of inline fragments that spread on User.

Example GraphQL query

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

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a 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.