Filter merge request to show only changes CODEOWNER is responsible to review
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### Problem to solve
Quickly filter the changes in the merge request to only show the files that you own.
### Intended users
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
* [Presley (Product Designer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#presley-product-designer)
### Further details
Let's say a new feature has been added and we have a merge request that needs to be approved by both Sasha and Presley where Sasha needs to approve almost everything but Presley only needs to approve the changes in the *.css and *.scss files.
In the CODEOWNERS document we can specify that all changes on *.css and *.scss files automatically assign Presley to approve the change.
But Presley does not have an easy way in GitLab to view the *.css and *.scss files.
### Proposal
Adding a filter to the changes tab in the merge request to only view the files that you're a codeowner of as specified by the CODEOWNERS document.
### Permissions and Security
### Documentation
### Availability & Testing
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What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change
See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning -->
### What does success look like, and how can we measure that?
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### What is the type of buyer?
Starter / Bronze and up as those can add CODEOWNERS
### Links / references
issue