[8.x] Enum casts accept backed values

[Propaganistas]

This PR builds upon #39315 to accept an enum's backed value as well.

Currently enum castable attributes first have to be instantiated (or be null) before they are accepted for casting:

enum MyEnum: string
{
    case Pending = 'pending';
}

$model = new Model;
$model->enum = MyEnum::Pending;

This can get verbose when handling requests:

// Request input = ['enum' => 'pending']

$request->merge([
   'enum' => MyEnum::from($request->input('enum')),
]);

$model->fill(
    $request->all()
);

By also accepting the backed values of an enum when casting, fluent model filling will be enabled (again).

$model = new Model;
$model->enum = 'pending';

// Request input = ['enum' => 'pending']

$model->fill(
    $request->all()
);

[Propaganistas]

This forces values to be valid a enum case.

[themsaid]

This looks ok to me. We do the same with date casting where we accept a date object or a string or an integer.