Add Support for ssh-agent for ssh authentication

[Anunayj]

Is your feature request related to a problem? Please describe.
Many users tend to use a ssh-agent for convenience/security purposes. As of right now pwncat requires user to pass the private key file for ssh authentication. which might not be always possible because

1. User uses a hardware token to store the ssh key and cannot possibly get the identity file.
2. User uses gpg-agent with ssh support. Which does not expose the private key as a file by default.

Describe the solution you'd like
Instead of asking for a identity file, ask the ssh-agent for authentication. paramiko already has support for it here

SSH-agent forwarding would be very nice to have too, which seems to be undocumented? https://gist.github.com/toejough/436540622530c35404e6
This should come with big warning texts though, since exposing your ssh-agent on a untrustworthy machine can only end in a disaster.

Alternative
Just generate another key for use with pwncat.

[Anunayj]

Also the current system assumes that the key in use is a RSA key, which might not be true in all cases, many users have begun to move to Ed25519 (very new, and kinda unsupported) or ECDSA (if you can get them to work on your computer****).

[trevorbryant]

@calebstewart I don't know if you currently have this on your radar, so pinging you back just in case.

[calebstewart]

With paramiko, the correct way to do this would be to use the SSHClient class instead of manually constructing the SSH session like I have been doing. I implemented this (as seen in #122), but can't merge this until the upstream is fixed (see paramiko/paramiko#1606).

Once the upstream is fixed, I will take the issue out of draft state and do some testing before merging. Hopefully that's soon.