Created reviewer dropdown for the reviewer drawer

What does this MR do and why?

Create a new reviewer dropdown that will show the approval rules eligible users by default until the user searches. When the user searches we fetch from the autocomplete API.

#448437 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

• Enable reviewer_assign_drawer
• Visit a merge request
• Click the Edit button in the reviewer widget