Add validation for user / group paths in CODEOWNERS file

Problem to solve

This feature proposal came up during a long running customer issue: https://gitlab.zendesk.com/agent/tickets/559894

In our situation, the group was using an incorrect path and had incorrect capitalization. There were no errors presented to indicate that the group was invalid, so it required a lot of time and effort to discover.

If a group path or username were to change in the future, this would also break the code owners rules. When this happens, approvals may be set to optional, bypassing expected merge request approvals.

User experience goal

Improve troubleshooting and setup of code owners

Proposal

We would like a feature to validate the CODEOWNERs file similar to how pipelines can be validated.

The user decides when to run it and on what branch. The validator would confirm if any users or groups cannot be found / cannot access the intended project.