Allow updating squash option per branch rule (excl. All branches)

What does this MR do and why?

Allows editing squash option on the branch rule details page

This excludes updating squash option for All branches + non-EE, see #512674 (comment 2341726245) for context.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Enable the branch_rule_squash_settings feature flag.
2. Ensure you're on an EE instance of GitLab.
3. Create a project in a group.
4. Create a protected branch for that project and add squash options .
   • I.e. FactoryBot.create(:branch_rule_squash_option, protected_branch: Project.last.protected_branches.create(name: 'test-branch'), project: Project.last)
5. Navigate to Project > Settings > Repository.
6. Click on View details next to the protected branch created in step 4.
7. On the Branch rule details page, the Squash commits when merging section should have an Edit button.
8. Clicking the edit button should open a drawer and editing should work as expected.
9. The Edit button should not be available when viewing branch details for All branches.
10. The Edit button should not be available when viewing branch details on a non-EE instance.

Related to #512674 (closed)