add example mTLS + proxy mode

[jxx-gg]

• The example includes 3 binaries, server, target and client.
  • server: it only serves as a proxy, and it doesn't create new sessions.
  • target: it registers at the server, and wait for incoming connections. If a supported incoming connection is received, it dials a pre-specified address.
  • client: it registers at the server, and tries to dial at the [dialTarget, dialTargetType]. Once a tunnel session is established (via server) to the target, it will connect the tunnel session to the local stdIO. This can be used with ssh's ProxyCommand option (see comments in the client.go for an example command).
• The example included two target type SSH and gNMI.
• mTLS support is also added.