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/socket_opts.rs

@@ -0,0 +1,19 @@
+//! Socket-level options for TCP streams (keepalive, etc.).
+//!
+//! Uses `socket2::SockRef::from()` to borrow the raw fd without ownership transfer.
+
+use std::io;
+use std::time::Duration;
+use tokio::net::TcpStream;
+
+/// Apply TCP keepalive to a connected socket.
+///
+/// Enables SO_KEEPALIVE and sets the initial probe delay.
+/// On Linux, also sets the interval between probes to the same value.
+pub fn apply_keepalive(stream: &TcpStream, delay: Duration) -> io::Result<()> {
+    let sock_ref = socket2::SockRef::from(stream);
+    let ka = socket2::TcpKeepalive::new().with_time(delay);
+    #[cfg(target_os = "linux")]
+    let ka = ka.with_interval(delay);
+    sock_ref.set_tcp_keepalive(&ka)
+}