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-http/src/lib.rs

@@ -3,6 +3,7 @@
 //! Hyper-based HTTP proxy service for RustProxy.
 //! Handles HTTP request parsing, route-based forwarding, and response filtering.
 
+pub mod connection_pool;
 pub mod counting_body;
 pub mod proxy_service;
 pub mod request_filter;
@@ -10,6 +11,7 @@ pub mod response_filter;
 pub mod template;
 pub mod upstream_selector;
 
+pub use connection_pool::*;
 pub use counting_body::*;
 pub use proxy_service::*;
 pub use template::*;