[7.x] Add AsMorphPivot trait

[idsulik]

This PR for 7.x #34488

The benefit of the trait is that you can add it to your custom model(which extends base Model, but not Pivot or MorphPivot model).
There is already exists AsPivot trait which is useful, so I add AsMorphPivot for morph pivot models

[taylorotwell]

We aren't adding new features to 7.x.