[12.x] Add possibility to define nested validation rules as array

[shaedrich]

This PR adds the possibility to define nested validation rules using array notation

Features

• Nested array to dot notation
• Automagically guessing whether the rule applies to an array or an object

Related features

• This is the declarative approach as compared to Rule::forEach()'s imperative approach introduced in [9.x] Add ability to generate unique validation rules per nested array element (repush) #40498 by @stevebauman

Is this a BC?

TL;DR
Yeah and no

It is already possible to have this syntax in your code, although unlikely due to it leading to unexpected results. So while these results would change, the syntax wasn't supported by the framework per se. So, I'd be happy to target master instead if this is actually considered a BC.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!