Support for additive only approval rule edits
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## Problem to Solve
Currently the setting to [`Prevent editing approval rules in merge requests`](https://docs.gitlab.com/ee/user/project/merge_requests/approvals/settings.html#prevent-editing-approval-rules-in-merge-requests) is all or nothing. When enabled, no changes can be made to merge request approval rules. When it's not enabled, organizations are able to create more flexibility, but they're unable to enforce a minimum set of required approvals.
## Proposal
We should introduce some flexibility to this control (or new control) by making the ability to edit rules additive _ONLY_. This would allow you to enforce the project configured rules, but then on a per MR basis evaluate if you might want to add additional rules.
### Additional Details
- https://gitlab.com/gitlab-org/gitlab/-/issues/396759#note_1317570912
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