Support username in assign push options

What does this MR do and why?

Support usernames in the merge_request.assign and merge_request.unassign push options. We documented support for this but this was never working.

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.

Related to #344276 (closed)