[9.x] Introduce Arr::prependKeysWith helper

[denjaland]

This PR introduces a new Arr helper function which allows you to easily prepend all key names of an associative array.

Example usage:

$array = ['key' => 'value'];
 
$array = Arr::prependKeysWith($array, 'prefix.');
 
// ['prefix.key' => 'value']

This is particularly helpful when you want to reuse FormRequest rules in other FormRequests.

E.g. Consider you have a CreateAddressRequest with the following rules:

class CreateAddressRequest extends FormRequest
{
    public function rules()
    {

        return [
            'streetAndNumber' => 'required',
            'zip' => 'required',
            'city' => 'required',
            'cc' => 'required',
        ];
    }
}

and you have a CreateAccountRequest which expects the information for the account, and the address as well; using the new helper, you can easily pull in the rules from the CreateAddressRequest class and prefix it:

class CreateAccountRequest extends FormRequest
{
    public function rules()
    {
        return array_merge([
            'name' => 'required',
        ], Arr::prependKeysWith(
            (new CreateAddressRequest())->rules(),
            'address.'
        ));          
    }
}

This will properly validate the nested address:

{
  name: 'John Doe',
  address: {
    streetAndNumber: 'Grand Place 123',
    zip: '1234',
    city: 'New York',
    cc: 'BE'
  }
}