feat: support forward and reverse HTTP/HTTPS proxy with basic authentication

[esrrhs]

Summary

This PR implements full support for forward and reverse HTTP/HTTPS proxies in SPP, complete with optional username/password Basic authentication.

Key Features & Implementation

1. HTTP/HTTPS CONNECT Tunneling:
   • Supports the HTTP CONNECT method for HTTPS and arbitrary TCP streams.
   • Responds with HTTP/1.1 200 Connection Established upon handshake.
2. Standard HTTP Request Proxying:
   • Transparently parses and proxies standard HTTP methods (GET, POST, PUT, DELETE, etc.).
   • Converts absolute URIs to origin-form relative paths.
   • Strips Proxy-Authorization and Proxy-Connection headers before sending to target servers to prevent credential leakage.
3. Optional Basic Authentication:
   • Matches SOCKS5 authentication behavior: if -username and -password are configured, proxy requests require valid Proxy-Authorization: Basic <base64> credentials.
   • Unauthenticated requests receive HTTP/1.1 407 Proxy Authentication Required.
   • If no username/password is configured, authentication is disabled.
4. Forward and Reverse Proxy Modes:
   • Forward HTTP proxy (-type http_client): client listens locally on fromaddr and proxies out via the remote server.
   • Reverse HTTP proxy (-type reverse_http_client): server listens on fromaddr and proxies requests into the client network.
5. Config Generator & Documentation:
   • -genconfig now generates config_http_client.json and config_reverse_http_client.json.
   • Updated README.md and USAGE.md with quick start commands and schemas.

Testing

• Added unit and end-to-end test cases in proxy/http_test.go:
  • TestHttp_AuthCheck: unit verification for Basic authentication logic.
  • TestE2E_HTTPProxy_Forward: verifies 407 challenge, CONNECT tunneling, and HTTP GET request forwarding.
  • TestE2E_HTTPProxy_Reverse: verifies reverse HTTP proxying and auth handling.
  • TestE2E_HTTPProxy_NoAuth: verifies unauthenticated forward proxying.
• All packages and existing E2E tests pass (go test ./...).