Expose CODEOWNERS rules via API

Proposal

Build a API endpoint (REST or GraphQL) to expose CODEOWNERS information.

This would be useful for integrations with external systems as well as to build summary information within GitLab itself, as noted on Parse CODEOWNERS into DB tables (#389091)

will allow for the of APIs that integrate permissions with branch rules for external consumption

We may want to expand the scope of this to include other permissions around branches and rules.