[9.x] Allow validator messages to use nested arrays

[ollieread]

This PR allows for validator messages to be nested without the dot notation.

Here's an example of the current way of providing custom messages.

[
   'attribute.rule1' => 'message1',
   'attribute.rule2' => 'message2',
   'attribute.rule3' => 'message3',
   'attribute.rule4' => 'message4',
   'attribute.rule5' => 'message5',
]

This PR makes it possible to nest the rule => message elements.

[
   'attribute' => [
       'rule1' => 'message1',
       'rule2' => 'message2',
       'rule3' => 'message3',
       'rule4' => 'message4',
       'rule5' => 'message5',
    ]
]

The idea is that it makes it easier to read and identify.

[siarheipashkevich]

@ollieread will it work for FormRequest attributes?

[ollieread]

@ollieread will it work for FormRequest attributes?

@siarheipashkevich that's specifically why I created it, working for all validators was a side effect.