repo-updater: Add rate limit registry.
Created by: ryanslade
The registry is responsible for keeping track of rate limiters per external service. It is created during startup in repo-updater is intended to be passed around as a dependency, see ChangesetSyncRegistry
as an example.
Initially added support for GitLab, other service types will be added in followup PRs
Part of: https://github.com/sourcegraph/sourcegraph/issues/8546