EventStore - Define EE-only subscribers

Problem

In !79291 (comment 824349792) we realized that the way we can define EE-only subscribers is not ideal since the subscriptions are only declared in Core (lib/gitlab/event_store.rb).

To enable a EE subscriber for now we need to create a no-op subscriber in Core and extend it in EE with actual work. This is not ideal because we would dispatch events to subscribers and the jobs are enqueued even though they do nothing.

Solution

We could define ee/lib/ee/gitlab/event_store.rb where we define EE-only subscribers and then extend the Core EventStore using prepend_mod_with.