Add allowed to merge drawer with users

What does this MR do and why?

Adds Drawer with possibility to add/remove users from allowed to merge setting Roles and groups will be added in the follow-up Undo option as well The changes are under the :edit_branch_rules FF

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

1. Enable edit_branch_rules FF
2. Go to Settings > Repository in any project
3. Expand branch rules
4. Add new rule with "Add branch rule" dropdown
5. Go to rule detail page and click on edit button in the "Allowed to merge" setting
6. Drawer should open on the right with possibility to add/remove users

Related to #438968