Manage CODEOWNER groups without being a member of that group
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### Release notes
When using [groups as a codeowners](https://docs.gitlab.com/ee/user/project/codeowners/#add-a-group-as-a-code-owner), there is no way that Maintainers/Owners manage the members of that group. Sometimes people are out of the office, personal time off, or for any other reason are not being able to approve/review the Merge Request during a certain period of time, therefore a significant blockage is preventing the process to move forward. If this situation comes up, as a workaround we could add other temporary person to that codeowner group who approves/reviews the changes meanwhile the codeowner comes back.
Additionally, if the codeowner who is on PTO belongs to more than one codeowner group, there is no way for Maintainers/Owners to determine all the groups that person belongs to.
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### Problem to solve
1. If codeowners are not being able to approve/review Merge Requests due to their absence, the process gets stuck because Maintainers/Owners cannot modify the codeowners group they belong in order to add another person to temporarily backup the main codeowner.
2. Maintainers/Owners do not have a comprehensive coverage about all the codeowner groups that a person belongs to.
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### Proposal
1. Maintainers/Owners could edit the members of codeowner groups for adding/removing users that for some reason are not being able to review/approve any Merge Request during a certain period of time.
2. Maintainers/Owners could propagate the temporary change of adding/removing a person to all groups where a codeowner belongs to.
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### Intended users
This change is mainly focused for team managers who need a temporary person to approve/review Merge Requests on behalf of people who are temporarily out of the office.
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### Feature Usage Metrics
The whole Merge Request process does not get stuck when any of the codeowners is not being able to review/approve the code for a certain period of time.
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### Does this feature require an audit event?
No
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