Implement assigned issues in GraphQL without project or group scope

Summary

Draft: [PoC] Allow querying issues by assignee... (!99980 - closed) implements a rough PoC of a GraphQL endpoint for returning the user's currently assigned issues, the equivalent of the https://gitlab.com/dashboard/issues?assignee_username=[username] endpoint shown when clicking the icon in the main top navigation bar.

This was developed to allow for the wider testing of using IndexDB and Service Workers to pre-fetch and cache assigned issues for a snappier user experience in POC: Snappy GL (!98444 - closed).

Proposal

Using this PoC as a precedent, the endpoint can now be properly implemented. Some things the PoC is missing:

• Unit tests
• A better method of setting the resource parent (see !99980 (comment 1122348241))
• A way to allow the user to be omitted entirely (see #377514 (comment 1143854617))

Requirements

• Can be used without group/project
• Implements sorts
• Implements filters

Example query

query {
  issues(assigneeUsername: [username]) {
    nodes {
      id
    }
  }
}