[5.4] Add ability to pass accepted values as argument list

[KKSzymanowski]

This adds ability to pass the allowed(or not allowed) values for in and notIn rules as argument list instead of an array(like Model::with()).

For example:

Rule::in('foo', 'bar', 'baz');

will be equivalent to:

Rule::in(['foo', 'bar', 'baz']);

The values, even if they are not necessarily strings, are being cast to strings in the implode call in Illuminate\Validation\Rules\In::__toString(), so I think the array|string parameter hint is OK. Another option would be to just call it mixed.