[5.6] Required If Rule based on a clousure or boolean value

[alfonsobries]

This PR will allow me to create a required rule-based in a user closure or a boolean value, the same behavior could be achieved with a custom closure but I think this provides a cleaner way to add some custom condition to require a value

Examples:

$this->validate($request->all(), [
   'role_id' => Rule::requiredIf(function () use ($request) {
   		return $request->user()->is_admin
   		   && $some_other_complicated_condition; 
    })
})

$this->validate($request->all(), [
   'role_id' => Rule::requiredIf($request->user()->is_admin)
})

[alfonsobries]

As a quick note the main idea is to avoid this type of conditions:
'role_id' => (auth()->user()->is_admin ? 'required' : '') . '|integer'

Better:
'role_id' => [Rule::requiredIf(auth()->user()->is_admin), 'integer']

[garygreen]

Surprised this got merged. It's just syntax sugar and yet another thing to maintain. Related to #25103

'role_id' => (auth()->user()->is_admin ? 'required' : '') . '|integer'

Better:

'role_id' => [Rule::requiredIf(auth()->user()->is_admin), 'integer']

Why is it better? Meh.

[garygreen]

Also this opens up a whole new door of PRs. Can we also add:

requiredWithout
requiredIfNot
...

This only makes sense to me if we converted all the existing rules into first class citizens, without the need to serialize the rules to a string again. But the way Laravel works at the moment is it essentially just creates a rule-based string, so there's no real benefit from just creating that string yourself. In fact it just slows down your validation.