aibridge: implement proxy chaining support

[ssncferreira]

RFC

Acceptance criteria:

• Added & used config values: CODER_AIPROXY_UPSTREAM, CODER_AIPROXY_UPSTREAM_CA
• CODER_AIPROXY_UPSTREAM specifies the upstream proxy URL (format: http://[user:pass@]host:port or https://[user:pass@]host:port)
• CODER_AIPROXY_UPSTREAM_CA specifies a path to a PEM-encoded CA certificate to trust for the upstream proxy's TLS interception (required when chaining through an SSL-bumping proxy)
• Both HTTP CONNECT tunnels and regular HTTP requests are routed through the upstream proxy
• Proxy authentication credentials in the upstream URL are properly forwarded
• Has tests