feat: add TCP keepalive options and connection pooling for improved performance

- Added `socket2` dependency for socket options.
- Introduced `keep_alive`, `keep_alive_initial_delay_ms`, and `max_connections` fields in `ConnectionConfig`.
- Implemented TCP keepalive settings in `TcpListenerManager` for both client and backend connections.
- Created a new `ConnectionPool` for managing idle HTTP/1.1 and HTTP/2 connections to reduce overhead.
- Enhanced TLS configuration to support ALPN for HTTP/2.
- Added performance tests for connection pooling, stability, and concurrent connections.

rust/Cargo.toml

@@ -88,6 +88,9 @@ async-trait = "0.1"
 # libc for uid checks
 libc = "0.2"
 
+# Socket-level options (keepalive, etc.)
+socket2 = { version = "0.5", features = ["all"] }
+
 # Internal crates
 rustproxy-config = { path = "crates/rustproxy-config" }
 rustproxy-routing = { path = "crates/rustproxy-routing" }