Source code rules - Move Merge request options to source code rules
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=357575)
</details>
<!--IssueSummary end-->
### Problem to solve
Once https://gitlab.com/gitlab-org/gitlab/-/issues/351468 is implemented, it might make sense to move additional features under source code rules and allow them to be managed on a per branch basis.
This issue is to investigate Merge request options.
- Merge method/Squash commits when merging
- Merge options
Note: [Status checks](https://gitlab.com/gitlab-org/gitlab/-/issues/357572) and [Merge checks](https://gitlab.com/gitlab-org/gitlab/-/issues/357571) have their own issues

### Proposal
- Determine if there is value in managing Merge options on a per branch basis
- Define a section within branch rules to manage Merge options
issue