A new model event after retrieved that fires after getting relations

[Luffyyy]

Hello.

Currently the 'retrieved' event is fired before relations are retrieved. While this is fine in many cases, sometimes we need to handle stuff with relations, for example, appending an attribute based on them using the relationLoaded method.

After looking around, this issue #29658 is mostly what I'm describing I believe.
And I think (unless I failed finding it) no one has followed up an idea for it. So yeah basically if we could have an event that fires right after relations are retrieved that would help with some situations.