chore: update proxy.go for WebSocket handshake

[saurabhraghuvanshii]

Notes for Reviewers

• The proxy now recognises any request that carries Upgrade: websocket and transparently turns itself into a WebSocket tunnel instead of doing a plain HTTP round-trip.
• A new helper (proxyWebsocket) upgrades the client connection.
• Normal HTTP traffic is still handled exactly as before.

This PR fixes #27

Signed commits

• Yes, I signed my commits.

[leecalcote]

@saurabhraghuvanshii, thanks for this. What test cases have you exercised with this change in place?