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)