Make requests modifiable v2

[tabacitu]

WHY

BEFORE - What was wrong? What was happening before this PR?

As reported in #3789, the FormRequest wasn't passed to the saving. So if you edited the request there, tough luck.

AFTER - What is happening after this PR?

The FormRequest gets passed to the saving.

HOW

Is it a breaking change or non-breaking change?

BREAKING

How can we test the before & after?

Do this in your FormRequest:

    protected function prepareForValidation()
    {
        // add something to the request
        $this->request->add(['updated_by' => backpack_user()->id]);
    }

BEFORE this PR you'll notice it's ignored, it doesn't reach the saving part (if you dd() before it gets saved).
AFTER this PR it does reach it.

Of course, then it will get stripped by getStrippedSaveRequest(), but that's a different problem, but you can do something like this then (and add updated_by to your validation rules as nullable):

    protected function prepareForValidation()
    {
        // tell Backpack to save the validated inputs (instead of all inputs that have fields)
        \CRUD::set('create.strippedRequest', function ($request) {
            return $request->validated();
        });

        // add something to the request
        $this->request->add(['updated_by' => backpack_user()->id]);
    }

[tabacitu]

Todo:

• @pxpm & @promatik - please let me know if you guys think this is worth doing; if the breaking change is worth it;
• @tabacitu - merge [4.2][Feature] Custom save request stripping #3987 (this PR depends on it)
• @tabacitu - add step in 4.2 upgrade guide for this - docs for saving FormRequest instead of request() docs#288
• @tabacitu - add docs on the recommended way to add an item to Create/Update without an input
• @tabacitu - reply to this StackOverflow issue with the new solution for 4.2

[promatik]

I approve the breaking change!
This makes it possible to edit the Request, I think it's a good think to have.

[pxpm]

I like this.

I think it makes this bit of functionality way way more easy to understand and work with.

No more store() and update() overrides to add a simple value in the request. 👍