SSH: Support executing commands and interactive shell with port forwarding

[GGonryun]

This PR enables

• interactive shells & port forwarding: p0 ssh <instance> -L 56789:80
• executing commands & port forwarding p0 ssh <instance> sleep 10 -L 56789:80

In order to support this behavior execution of the port forwarding command is done in a child process. The child process uses streams to intercept and merge the child processes (and it's descendent's) output into the parent processes stdout.

[linear]

ENG-1653 p0 cli SSH: Support executing commands with port forwarding

The -L option for SSH normally allows users to specify a command to run while port forwarding, but The implementation in ENG-1594 currently does not support executing an interactive command with port forwarding.

Possible Complications

1. The current SSM command for opening a port requires a different session type (Port) and uses a different AWS managed document (StartPortForwardingSession). It might be necessary to execute two different commands to get port forwarding working with an interactive command.

Definition of Done

1. Investigate both the behavior of ssh <destination> <command> -L 80:80 and the behavior of the -nL option and try to bring our p0 ssh command inline with the implementatoin.
2. Allow users to execute a command and port forward at the same time. This would follow the syntax: `p0 ssh [command [args..]] -L port:port`

[nbrahms]

Can you double-check the functionality of:

• Typing exit in the SSH session
• Typing CTRL-d in the SSH session
• Resizing the SSH window (especially in a vi or nano window)

[nbrahms]

Doesn't this need to be ["inherit", "inherit", "pipe"] in order for the pty to be preserved?

[nbrahms]

I think this needs to be different for the ssh session and the port-forward subprocesses.

[GGonryun]

If this is ["inherit", "inherit", "pipe"] then I get a subprocess with no readable output stream, this means that we can't suppress the "Starting Session" message we get when SSM commands run which results in this output:

[GGonryun]

If we don't mind getting multiple "starting session" messages, then we can switch to ["inherit", "inherit", "pipe"]. What do you think?

[nbrahms]

It looks like this is dropping stderr?

[GGonryun]

I don't think it should be dropping stderr. From my understanding i'm piping the original output into a proxy stream so I can process the messages and then passing it back to process.stdout, which should leave stderr unaffected.

[GGonryun]

• Resizing the SSH window (especially in a vi or nano window)

Everything works fine except nano doesn't display properly:

Currently on Main

This PR

[GGonryun]

Closing this PR, an alternative approach is being taken in #50 which does not used detached processes and also doesn't intercept the primary process' output stream