add telnet support

[ferki]

ref

Currently it's not possible to use telnet with rex, but the implementation is modular enough that it might possible to add (thinking architecturally), via writing proper Rex::Interface::{Connection,Exec,File,Fs}::Telnet modules. I'm not sure how file operations would exactly work over a telnet connection, and I don't see how big obstacle would that be (and quite possibly I'm just missing something obvious).

Based on a quick search, looks like Net::Telnet (https://metacpan.org/pod/Net::Telnet) would be a good start to interact with machines over telnet connections when using Perl, and quite surely rex would use that module in the background if implemented.