Feat: Better SSH Socket Support via ssh-agent (Linux Host)

[drmikecrowe]

what

This fixes firing up ssh-agent properly by testing two conditions:

• First, attempt to list added keys using ssh-add -l &>/dev/null. If this returns an error of 2, we can't communicate with the agent
• Next, load the environment (if it exists) and repeat the above test.
  If these fail, then restart the ssh-agent

Additionally, only map SSH_AUTH_SOCK from a real folder. /tmp will not properly map into docker and results in being unable to access the socket.

Finally, if on Linux and using SSH_AUTH_SOCK and you are not running as root (an upcoming PR), you have to map the host socket using socat to access the host folder as root.

why

• ssh-agent functionality is not robust enough (see above)

[korenyoni]

Deferring to @Nuru because this affects high-level Geodesic wrapper functionality.

[drmikecrowe]

@korenyoni -- I'm going to close this one as it's related to #594 and how that fix will impact the SSH socket