Add branch protections dropdown

What does this MR do and why?

Adds a branch protections dropdown as part of #362212 (closed)

⚠ changes are behind the branch_rules feature flag.

MR breakdown

In an attempt to keep the MRs small and easy to review I've broken it down as follows:

title	note
Add branch protections dropdown	👈 this MR
Ability to add branch protections	follow-up

Screenshots or screen recordings

How to set up and validate locally

1. Enable to branch_rules feature flag.
2. Navigate to <project>/-/settings/repository/branch_rules

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 #362212 (closed)