Proxy ssh-agent requests by setting a temp SSH_AUTH_SOCK for ssh remote servers

[jameshartig]

This is similar to #3067 but not specifically devcontainers. I work primarily in a remote workstation over SSH which I have connected in T3 via the Remote Environments. Locally I run 1Password with SSH Agent support. In VSCode/Cursor I can open a terminal and it proxies SSH Agent requests automatically by setting a temporary SSH_AUTH_SOCK for that terminal to a proxy socket which forwards requests to my local machine. I also have ForwardAgent yes set in my .ssh/config so even regular SSH works similarly. When I open a terminal in T3, that doesn't happen so anything requiring SSH keys (ssh, git, etc) doesn't work because there's no SSH_AUTH_SOCK.

I'd like T3 to create an ephemeral socket and set it as SSH_AUTH_SOCK inside the remote SSH terminal and proxy/forward SSH agent requests to my local SSH agent. This will make terminals in T3 work just like regular SSH and IDEs.

[jameshartig]

Oh this might be solved by #8190.