Add /server-ip endpoint to return server's local IP address

[paimus]

This change introduces a new /server-ip endpoint that returns the IP address of the server (or Pod in Kubernetes) handling the request, as opposed to the existing /ip endpoint which returns the client's IP address.

Changes:

• Add getServerIP() helper in helpers.go to detect server's primary IP
  • Prefers non-loopback IPv4 addresses
  • Falls back to IPv6 if no IPv4 found
  • Uses UDP dial as last resort for IP detection
• Add ServerIP handler in handlers.go
• Register /server-ip route in httpbin.go
• Add serverIPResponse struct in responses.go

This is useful for debugging and service discovery in containerized environments where the server's IP may need to be identified.

[mccutchen]

I appreciate the contribution, but as noted in https://github.com/mccutchen/go-httpbin/issues/113#issuecomment-1466225358 I am reluctant to expose this potentially sensitive info about internal networks by default.

I'd consider a change that only exposed this endpoint if it was explicitly enabled, along the lines of the USE_REAL_HOSTNAME option added in https://github.com/mccutchen/go-httpbin/pull/81.

That being said, I'm not sure about the implementation here. Why is IPv4 preferred over IPv6? Also I don't think making a throwaway UDP connection (why to port 80? does the port just not matter here?) is a reasonable thing to do.