wasihttp: implement basic server response handling

[ydnar]

Implement basic wasi:http/proxy server response handling.

Depends on tinygo-org/tinygo#4555.

TODO

• Implement consistent pattern for conversion functions between net/http and wasi:http types.
• Implement http.ResponseWriter.
• Proxy optimizations to wire incoming request to outgoing request, and incoming response to outgoing response.