Describe your problem or use case

There are still different ways to unregister various callback mechanism throughout the C-API.

Previously realm_add_realm_changed_callback and realm_add_schema_changed_callback have been updated in https://github.com/realm/realm-core/issues/5217 to follow the C-API managed approach of returning a token object that could be released through realm_release to unregister the callbacks.

There are still numerous use cases where the C-API has both a x_register and a x_unregister method, which is annoying as we cannot just manage all C-API allocated resources in a common way.

Identified APIs:
https://github.com/realm/realm-core/blob/master/src/realm.h#L3853
https://github.com/realm/realm-core/blob/master/src/realm.h#L3938
https://github.com/realm/realm-core/blob/master/src/realm.h#L3961

Describe the solution you'd like

Would be nice if all callback registrations could just follow the same pattern of releasing resources by calling realm_release with a token object issued when registrating the callback. Since all these callbacks are usually not on the hot path, I don't see an issue in the additional allocation.