Add approval rules drawer component (project settings)

What does this MR do and why?

Adds an approval rules drawer component to Project > Settings > Merge requests > Merge request approvals as part of updating our frontend to use a Drawer component instead of a Modal everywhere approval rules can be managed.

Issue: #439397 (closed)

Note: changes in this MR are behind the :approval_rules_drawer feature flag.

Implementation breakdown:

Description	Issue/MR
Add approval rules drawer component (Project > Settings > Merge requests)	This MR
Add approval rules drawer component (Merge Request > New/Edit) Address docs changes (!146502 (comment 1807508724))	follow-up MR
Add approval rules drawer component (Branch rules > Edit)	follow-up MR
Update approval rules form to use User and Group selectors (mockups)	follow-up MR
Rollout approval_rules_drawer feature flag	#444628 (closed)

MR acceptance checklist

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Screenshots or screen recordings

Before	After
(Add)	(Add)
(Edit)	(Edit)
(Enable coverage check)	(Enable coverage check)

How to set up and validate locally

1. Enable the approval_rules_drawer feature flag.
2. Navigate to Project > Settings > Merge requests > Merge request approvals.
3. Add, Edit, and Enabling coverage checks should now happen in a drawer instead of a modal.

Related to #439397 (closed)