Filter Merge Requests by CODEOWNERS group
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### Problem to solve
When using [Codeowners](https://docs.gitlab.com/ee/user/project/codeowners/), we can set members of a [group or subgroup](https://docs.gitlab.com/ee/user/project/codeowners/#add-a-group-as-a-code-owner) as codeowners. For example:
```
# All group members as Code Owners for a file
file.md @group-x
# All subgroup members as Code Owners for a file
file.md @group-x/subgroup-y
```
At the moment, we can [filter Merge Requests](https://docs.gitlab.com/ee/user/project/merge_requests/#filter-the-list-of-merge-requests) using `Approver` and `Reviewer`. This includes the members of group/subgroup defined in the CODEOWNERS however we cannot directly use the group in the filter.
### Proposal
We should allow the filter to be able to search using the groups or subgroups defined in the CODEOWNERS file.
### Intended users
* [Parker (Product Manager)](https://about.gitlab.com/handbook/product/personas/#parker-product-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/product/personas/#delaney-development-team-lead)
* [Presley (Product Designer)](https://about.gitlab.com/handbook/product/personas/#presley-product-designer)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/product/personas/#sasha-software-developer)
### Feature Usage Metrics
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