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-config/src/proxy_options.rs

@@ -208,6 +208,10 @@ pub struct RustProxyOptions {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub connection_rate_limit_per_minute: Option<u64>,
 
+    /// Global maximum simultaneous connections (default: 100000)
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub max_connections: Option<u64>,
+
     // ─── Keep-Alive Settings ─────────────────────────────────────────
 
     /// How to treat keep-alive connections
@@ -272,6 +276,7 @@ impl Default for RustProxyOptions {
             enable_randomized_timeouts: None,
             max_connections_per_ip: None,
             connection_rate_limit_per_minute: None,
+            max_connections: None,
             keep_alive_treatment: None,
             keep_alive_inactivity_multiplier: None,
             extended_keep_alive_lifetime: None,