Support selecting CODEOWNERS section to be a required approval rule

Problem to solve

Even when #12137 (closed) is implemented, all code owner rules must be enforced or none. This means that if there are important compliance rules that must be enforced, every rule must be enforced. This means another team that simply wants to be notified and added as optional approvers of relevant merge requests must forgo using code owners or become a required approver.

Further details

For example:

• the compliance team needs to make sure all changes to the app/payments code is reviewed by Jane
• there are multiple development teams that work on the payment code, and want to automatically assign code reviews to the correct teams, app/payments/checkout, app/payments/renewal and app/payments/cancellation

If the approval for app/payments/checkout is actually optional, they will still be required to given an approval.

Proposal

Allow selecting specific CODEOWNERS sections when requiring CODEOWNERS approvals.

Mockup: