Consider user permissions when validating Code Owners

What does this MR do and why?

Consider user permissions when validating Code Owners

• Check that references to non-group names are users in the project
• Check that references to users have adequate access to approve

The result of these checks will be used in: !179627 (merged)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #502583 (closed)

MR acceptance checklist

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