authz: implement `PermsFetcher` interface for GitLab authz providers

Created by: unknwon

This PR implements PermsFetcher interface for GitLab authz providers, both SudoProvider and OAuthProvider.

Notes

• Tests will be added in followup PRs once I run through the new permissions syncing process end-to-end.
• This PR does not change any functionality currently in use, safe to merge upon approvals.
• This PR explicitly mentions in PermsFetcher definition that implementations should return "partial but valid results in case of error". The rationales are:
  • API calls to code host are expensive.
  • When a new user or repo just being added, there is literally no permissions, something (valid) is better than nothing. LMK any objections, if anyone thinks this will make debugging harder.

Implements #8492.