PoC proxy with stream support

[ngxson]

In this PR, the /v1/chat/completion handler is removed and replaced by a proxy function that will proxy the request to /chat/completion endpoint.

This acts as a demo on how the streamed proxy function can work.

Currently, it works with web UI. But it crashes when the request is terminated unexpectedly (i.e. user clicks on Stop button on web)

Tested so far these cases:

• normal case (both ends of the proxy open and terminate gracefully)
• the destination server does not exist --> return 500 code to client
• the client terminates unexpectedly (for example, user clicks on Stop on webui)
• the destination server terminates unexpectedly

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch xsn/poc_proxy_2

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