WIP (DON'T MERGE): apt_key type/provider

Started working on a type and provider for ``apt_key`` to replace the current ``apt::key``.

It's the first time I'm trying this. Basically read through the Types and Providers book yesterday and then came up with this. I think I got all the params right and that there are no properties but, not sure. I'm also not entirely convinced about my ``autorequire`` but I think it makes sense.

I purposefully decided to not use the same attribute names as ``apt::key`` as I find ``key_id``, ``key_server`` etc fairly redundant on a type called ``*_key``.