updated most of Rsync with server-specific auth

As it was, the Rsync module/command was taking most of its config directly from `Rex::Config`, when `get_current_connection->{server}` is a `Rex::Group::Entry::Server` that has proxy methods for most of what it was using, so now it supports group-specific/server-specific auth for the most part.

I think the only notably missing method during my light conversions here was `get_password_auth`, but I'm not sure it's even supported to have password auth globally and private/public key auth for just one group (or vice versa).