enhancement suggestions for pipes

[SomajitDey]

• Custom piping server URL can be set by private peer with x-piping-server header. Public peer no more needs to know the piping-server address.
• Detect and pass visitor's-public IPv4/IPv6 as well as public port as x-your-ip headers; this acts like STUN. This helps with dynamic public IP too.
• Detect and pass private peer's public IPv4/IPv6 as well as public port to public peer as x-peer-ip header along with the redirect reply, provided x-peer-ip: masked is not opted for by the private peer. This helps with dynamic public IP too.
• Relay any message from private peer to public peer as x-peer-message header in the redirect reply to the public peer. WebRTC SDP offers may be sent like this, along with initial ICE candidates. This field/header is open for user applications to use anyway they please.
• Also, private peer may issue a redirect URL as x-redirect-peer-to header while making the piping request. In that case, the public peer will be redirected to this address. Private peer won't be redirected. Useful for use with localtunnel or localhost.run like services that expose the localhost to the internet with session-specific public sub-domains.