[8.x] Add Conditionable Trait to Relation

[nedwors]

Hi!

Thank you for all the amazing work 🔥

Overview

This PR adds the new Conditionable trait to the base Relation class.

Currently, the following would not work for a relationship query...

$user->posts()->when($shouldOrder, fn ($query) => $query->orderByPivot('..'))->get()

...because the Relation class does not have a native when/unless. So, it forwards it to the Builder but then this does not have access to the Relation's methods - in this case orderByPivot.

Compatibility

The only thought is that this could be a breaking change in the sense that some relations override some Relation methods - eg. get in BelongsToMany. So if any apps do something along the lines of...

$user->posts()->when($shouldPaginate, fn ($query) => $query->paginate(), fn ($query) => $query->get())

...the get method would now be elevated from the Builder up to the Relation?

Tests

Testing wise, I appreciate there is only one test. I felt it illustrated the feature nicely using an existing set up. But very happy to add more if desired.

[taylorotwell]

Reverted due to breaking tests on master.