Enable selection of groups for protected branches
### Problem to solve
Currently, only roles and distinct users can be whitelisted on protected branches. It would be very convenient if groups can be whitelisted for protected branches. Groups can be currently whitelisted for MR approvals; the same would ideally be applied to protected branches.
### Intended users
GitLab/group/project administrators
### Further details
Use case: we have a group of "approved mergers" who are the only ones allowed to merge MRs for certain projects (this helps to enforce code freezes). Whitelisting each individual user for every project is a pain, whereas being able to create a group of approvers, and whitelisting the respective group, would be much more streamlined.
### Proposal
Add "Groups" as a filter in the "Allowed to merge" and "Allowed to push" dropdowns within the "Protected Branches" section of a project's settings.
### Permissions and Security
Same as managing protected branches
### Documentation
Update this section to describe how groups can also be whitelisted
https://docs.gitlab.com/ee/user/project/protected_branches.html#using-the-allowed-to-merge-and-allowed-to-push-settings
### Testing
No extraneous testing requirements. Just need to validate that groups can be whitelisted for protected branches.
### What does success look like, and how can we measure that?
See above
### What is the type of buyer?
Starter and above
### Links / references
issue