[7.x] Add HasCasterClass interface

[brendt]

This PR adds an interface which allows castable types to specify their caster class. This approach has two advantages:

• Packages can provide caster classes with less configuration required from the user
• The casting configuration on models can now tell what class a property will be casted to, instead of what caster will be used.

As an example:

// Instead of this
class ModelX extends Model
{
    protected $casts = [
        'data' => CastToDTO::class . ':' . MyDTO::class,
    ];
}

// You could do this
class ModelY extends Model
{
    protected $casts = [
        'data' => MyDTO::class,
    ];
}

The underlying MyDTO class would look like this:

class MyDTO implements HasCasterClass
{
    public static function getCasterClass()
    {
        return CastToDTO::class . ':' . static::class;
    }
}

As you can tell from the example, my use case is with our DTO package. I would prefer our users to specify what type of DTO they want to cast to, instead of having to manually specify CastToDTO::class . ':' . MyDTO::class for each cast. The getCasterClass would be done in our abstract DataTransferObject implementation, and the user wouldn't have to worry about it.

[Gummibeer]

I love this! But for https://github.com/spatie/laravel-enum we would need to be able to pass additional information too.

MyEnum::class.':nullable'
EnumCaster::class.':'.MyEnum::class.',nullable'

So the interface method should accept additional arguments.

[brendt]

@Gummibeer you'll return whatever string you like from getCasterClass, so that wouldn't be a problem I believe.

I might send a new PR after this one, which would allow to pass pre-configured instances of casters, instead of their class names. But that's after this.

[brendt]

Hmm, thinking about it, that might not be a solution for your problem…

[Gummibeer]

I can return it - but it should also be possible to pass them in. So the method should accept all additional arguments, like the caster constructor.
I think that an array or the spread operator should be sufficient for this case!?

[brendt]

Gotcha. I will make sure $arguments are passed too, but I'll await Taylors feedback first, if he asks for additional changes, I can tackle them at once.

[taylorotwell]

Yeah I think the additional arguments would definitely be needed in cases. Other than that I don't see any major issues.

[brendt]

I'll make these changes next week, I'm a little swamped right now with client work :)

[brendt]

I found some spare time. Attributes that are given to a HasCasterClass object are automatically passed to the underlying caster class.

[slavarazum]

I think we should have an ability to work with custom attributes in better way.

Class instances suits much better here:

Array from casts method can be merged with array from $casts property. So, we can choose from 2 options or use both.

What do you think?

[brendt]

@slavarazum I think it's a good feature, but outside the scope of this PR. I plan on looking it this after this feature is merged

[brendt]

@taylorotwell could you take a look at my final changes? Any more remarks?

[taylorotwell]

Renamed HasCasterClass to Castable and getCasterClass to castUsing.

[taylorotwell]

May want to PR the docs for this.

[brendt]

Docs PR: laravel/docs#5934