initial release: Rust port of MasterHttpRelayVPN apps_script mode

Faithful port of @masterking32's MasterHttpRelayVPN. All credit for
the original idea, protocol, and Python implementation goes to him.

Implemented:
- Local HTTP proxy (CONNECT + plain HTTP)
- MITM with on-the-fly per-domain cert generation via rcgen
- CA auto-install for macOS / Linux / Windows
- Apps Script JSON relay, protocol-compatible with Code.gs
- TLS client with SNI spoofing (connect to Google IP, SNI=www.google.com,
  inner HTTP Host=script.google.com)
- Connection pooling (45s TTL, max 20 idle)
- Multi-script round-robin for higher quota
- Header filtering (strips connection-specific + brotli)
- Config-driven, JSON schema matches Python version

Deferred (TODOs in code):
- HTTP/2 multiplexing
- Request batching / coalescing / response cache
- Range-based parallel download
- SNI-rewrite tunnels for YouTube/googlevideo
- Firefox NSS cert install
- domain_fronting / google_fronting / custom_domain modes
  (mostly broken post-Cloudflare 2024, not a priority)

13 unit tests pass, 2.4MB stripped release binary.

config.example.json

@@ -0,0 +1,12 @@
+{
+  "mode": "apps_script",
+  "google_ip": "216.239.38.120",
+  "front_domain": "www.google.com",
+  "script_id": "YOUR_APPS_SCRIPT_DEPLOYMENT_ID",
+  "auth_key": "CHANGE_ME_TO_A_STRONG_SECRET",
+  "listen_host": "127.0.0.1",
+  "listen_port": 8085,
+  "log_level": "info",
+  "verify_ssl": true,
+  "hosts": {}
+}