[10.x] Add methods to determine if API resource has pivot loaded

[ziadoz]

I've refactored the trait code that checks if an API resource has a pivot here into a separate method, because it makes it easier to do complex checks in a Resource, which is something I've found myself needing to do:

$this->mergeWhen(
    $this->resource->field === 'whatever' && ! $this->resource->hasPivotLoaded('pivot_table'),
    ['foo' => 'bar'],
);

The changes here are covered by existing tests that use PostResourceWithOptionalPivotRelationship.