Support pluggable callbacks for entity registration

[thll]

This pull request replaces the EntityOwnershipService with a new EntityRegistrar across multiple classes in the codebase. Entity ownership registration moved to a new EntityOwnershipRegistrationHandler class.

This allows plugins to contribute custom handlers that are called when an entity is registered or unregistered.

/prd https://github.com/Graylog2/graylog-plugin-enterprise/pull/11094