[12.x] Validator default attributes formatter

[Jasonej]

This PR adds setDefaultAttributesFormatter to allow for control over how attribute names are formatted in cases where the default str_replace('_', ' ', Str::snake($attribute)) logic is not desired.

This allows for something like:

Validator::make([], ['data.someAttribute' => 'required'])
    ->setDefaultAttributesFormatter(fn($attribute) => $attribute)
    ->validate()

Which would produce:

The data.someAttribute field is required.

Instead of:

The data.some attribute field is required.

This doesn't break existing behavior because it only applies when a defaultAttributesFormatter has been set on the validator.