aibridge: implement core MITM proxy

[dannykopping]

RFC

Acceptance criteria:

• Runs as an in-process daemon, architected to be capable of running separately (i.e. no use of coderd globals)
• Added & used config values: CODER_AIPROXY_ENABLED, CODER_AIPROXY_LISTEN_ADDR, CODER_AIPROXY_CERT_FILE, CODER_AIPROXY_KEY_FILE
• Can receive & decrypt HTTP requests
• CLI tool works using proxy
• Has tests