RFC 503: Update PermSyncer to populate sub_repo_permissions table

Created by: ryanslade

The PermSyncer is responsible for calling out to each authz provider, which are the source of truth for permissions. We should use this information to persist sub repo permissions in our new sub_repo_permissions table.

We may need to create a stub interface until the auth providers have implementation that can provide sub repo permission information.

Requires https://github.com/sourcegraph/sourcegraph/issues/26634