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/crates/rustproxy-passthrough/src/lib.rs

@@ -11,6 +11,7 @@ pub mod tls_handler;
 pub mod connection_record;
 pub mod connection_tracker;
 pub mod socket_relay;
+pub mod socket_opts;
 
 pub use tcp_listener::*;
 pub use sni_parser::*;
@@ -20,3 +21,4 @@ pub use tls_handler::*;
 pub use connection_record::*;
 pub use connection_tracker::*;
 pub use socket_relay::*;
+pub use socket_opts::*;