[11.x] Dynamic Model Attribute Mutators

[tylernathanreed]

This PR adds the ability to dynamically register model attribute mutators without having to declare them as methods.

See #50759 for a scenario where this would be useful.

Ideally, on the boot method of a model, or an external trait, these mutators could be added:

class User extends Model
{
    public static function boot()
    {
        /** @var array<string,Attribute> */
        $attributes = [
            'example' => Attribute::make(...),
            /* ... some dynamic source ... */
        ];

        foreach ($attributes as $name => $attribute) {
            static::addDynamicAttribute($name, $attribute);
        }
    }
}

// echo $user->example;
// $user->example = 5;

[crynobone]

Marked as draft since tests are failing.

[driesvints]

Feel free to resend this one with passing tests, thanks.

[tylernathanreed]

Superseded by #50770