Add --vscode flag to ssh connect command

[alexellis]

Inspired by Ivan's recentl labctl CLI:

https://github.com/iximiuz/labctl/blob/fa58e59280fe07cf9725de034d5fab8d4097f93f/cmd/sshproxy/sshproxy.go#L145

Add --vscode flag to ssh connect command, to automate the command to launch VSCode pointed at a remote SSH session with actuated.

Using the instructions at https://code.visualstudio.com/docs/remote/ssh, I was able to launch a remote VSCode session and open the filesystem of the runner.

But automating this is in line with the current ssh connect command, so would potentially work better.

Having VSCode connected to a remote filesystem would be very handy for debugging the runner environment, which is what the "actuated-cli ssh connect" command is all about.