Allow user to use ssh key agent for ssh transport auth

[stormbard]

Use Case

Users may use ssh key agents with the auth key in it. Or they have their ssh key on an external security card; like Yubikey, and that is accessed via the key agent.

Describe the Solution You Would Like

The default ssh transport does not seem to utilize or expose an option to use an agent. I would like to be able to use my key agent.

Describe Alternatives You've Considered

I've used native-ssh as the transport and it works fine. This is a stop gap solution but since this is a new and experimental it might not work if things change in future version.s