[8.x] Add withOnly method

[JamesFreeman]

I have a model, that has many $with and new relationships are added frequently. These relationships are used throughout 95% of the customer-facing websites. However, for certain sections of the Admin system, where only a few relationships need to be loaded.

Imagine you have a model like this:

class Product extends Model{
    protected $with = ['prices', 'colours', 'brand'];

    public function colours(){ ... }
    public function prices(){ ... }
    public function brand(){ ... }
}

At the moment, I would only be able to achieve this by doing:

Product::without(['prices', 'colours'])->get();

When adding future relationships to my base model, they would be included by default, it would be nice to be able to do something like:

I have added a new method called withOnly (I couldn't think of a better name - any suggestions?) - which will exclude all the base $with and only add the relations that you specify like the following:

Product::withOnly(['brand']);

If you are happy to merge this in, I'll send a PR to update the docs repo. 👍

Thanks,
James.