[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.