Pass provider configuration into get

[lutter]

With quite a few providers, get can only retrieve resources if it is given additional parameters; e.g., ssh_authorized_keys needs a user or target parameter.

Make it possible to mark some attributes as config which means they need to be passed to the get operation, too.

There's also an interaction with what should be considered the primary key of a resource: some config parameters (e.g. user or target) should be used as part of the pk; in other situations, e.g., when a password needs to be provided to retrieve a resource, that password should not be part of the pk.