Implement public MR-level approval rules API

What does this MR do?

Add API endpoints so API users can list, create, update and delete MR-level approval rules.

The following API endpoints are added:

• GET /projects/:id/merge_requests/:merge_request_iid/approval_rules
• POST /projects/:id/merge_requests/:merge_request_iid/approval_rules
• PUT /projects/:id/merge_requests/:merge_request_iid/approval_rules/:approval_rule_id
• DELETE /projects/:id/merge_requests/:merge_request_iid/approval_rules/:approval_rule_id

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32133.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

#12055 (closed)