Add initial version of sshmux into inlets

Adds:

• inlets-pro sshmux server - mux one or many SSH servers behind a single TCP tunnel, including support for Proxy protocol v2 to obtain the remote IP addresses
• inlets-pro sshmux connect a helper for ssh -o ProxyCommand to encapsulate a TLS server name in the connection

Learn how to use sshmux here: https://inlets.dev/blog/2024/02/05/access-all-your-ssh-servers-with-sshmux.html