[4.x] REST API Globals return resolved relations

[martink635]

This resolves: #8462

Calling the REST API for globals resolves any given relationship, so it behaves the same as other API endpoints (entries, …).

This is a breaking change since it changes the behaviour of the public facing API of Statamic.

[jasonvarga]

It's a shame to have to make people wait until v5 for this fix, so we can make it opt in.

Throw this in your AppServiceProvider@boot:

\Statamic\Http\Resources\API\GlobalSetResource::withRelations();

In v5 can we make it the default behavior.