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/src/lib.rs

@@ -221,6 +221,9 @@ impl RustProxy {
                 .iter()
                 .filter_map(|s| s.parse::<std::net::IpAddr>().ok())
                 .collect(),
+            keep_alive: options.keep_alive.unwrap_or(true),
+            keep_alive_initial_delay_ms: options.keep_alive_initial_delay.unwrap_or(60_000),
+            max_connections: options.max_connections.unwrap_or(100_000),
         }
     }