[9.x] Introduce Invokable validation classes

[timacdonald]

This PR aims to introduce a new class based validation implementation that mixes the brevity + simplicity of Closure based rules with the shareable, extendable, and chainable nature of class based rules by introducing an Invokable validation rule.

Some additional notes:

• Brings 1st party translation support to failure messages, without the need to reach for the translation helper.
• You no longer need to manage state around messages and failures when it comes to complex validation where multiple validation errors may occur.
• Within a complex validation rule, you can no longer add a messaged and return true. Adding a messages causes the rule to fail - so no more double handling.
• Having the error messages managed in another layer also creates consistency across validation rules.
• An exception is thrown when it is unable to find a validation key, if $fail('key')->translate() is called. I've see this a bit where validation messages are not added. This makes it strict about finding the validation message.

Below is a comparison of the two implementations with a rule that handles a rich object...

class Quantity implements Rule
{
    protected $messages = [];

    public function passes($attribute, $value)
    {
        if (! is_array($value)) {
            $this->messages[] = trans('validation.quantity.must_be_an_object');

            return false;
        }

        if (! array_key_exists('magnitude', $value)) {
            $this->messages[] = trans('validation.quantity.missing_magnitude');
        }

        if (! array_key_exists('units', $value)) {
            $this->messages[] = trans('validation.quantity.missing_units');
        }

        return $this->messages === [];
    }

    public function message()
    {
        return $this->messages;
    }
}

class InvokableQuantity implements InvokableRule
{
    public function __invoke($attribute, $value, $fail)
    {
        if (! is_array($value)) {
            return $fail('validation.quantity.must_be_an_object')->translate();
        }

        if (! array_key_exists('magnitude', $value)) {
            $fail('validation.quantity.missing_magnitude')->translate();
        }

        if (! array_key_exists('units', $value)) {
            $fail('validation.quantity.missing_units')->translate();
        }
    }
}

[tpetry]

I would love to get rid of the boilerplate of passes() and message() just to have a failure-specific error messages. Really like the approach of also using $fail()!