Expose registry notifications config

Summary

The upstream registry supports configuration of webhook notifications: https://docs.docker.com/registry/notifications/#configuration

This is something we already support configuring in omnibus, but we don't provide a way to configure them in the charts.

This is needed for adding some usage tracking on gitlab.com gitlab-org/gitlab#20378 (closed)

And is related to an Distribution team task discussion. gitlab-org/distribution/team-tasks#608 (closed)