Draft: [PoC] Allow querying issues by assignee without group or project

What does this MR do and why?

This MR is intended to be closed when complete and not merged. This is a Proof of Concept (PoC) for the introduction of a GraphQL endpoint that allows querying issues by assignee without the use of group or project.

Usage

query{
  user(id: "gid://gitlab/User/1") {
    
    assignedIssues(state:opened, sort:CREATED_DESC) {
      nodes {
        id
        title
      }
    }
  }
}

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.