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/Cargo.lock

@@ -509,7 +509,7 @@ dependencies = [
  "hyper",
  "libc",
  "pin-project-lite",
- "socket2",
+ "socket2 0.6.2",
  "tokio",
  "tower-service",
  "tracing",
@@ -971,6 +971,7 @@ dependencies = [
  "rustproxy-metrics",
  "rustproxy-routing",
  "rustproxy-security",
+ "socket2 0.5.10",
  "thiserror 2.0.18",
  "tokio",
  "tokio-rustls",
@@ -1019,6 +1020,7 @@ dependencies = [
  "rustproxy-routing",
  "serde",
  "serde_json",
+ "socket2 0.5.10",
  "thiserror 2.0.18",
  "tokio",
  "tokio-rustls",
@@ -1204,6 +1206,16 @@ version = "1.15.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
 
+[[package]]
+name = "socket2"
+version = "0.5.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e22376abed350d73dd1cd119b57ffccad95b4e585a7cda43e286245ce23c0678"
+dependencies = [
+ "libc",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "socket2"
 version = "0.6.2"
@@ -1329,7 +1341,7 @@ dependencies = [
  "parking_lot",
  "pin-project-lite",
  "signal-hook-registry",
- "socket2",
+ "socket2 0.6.2",
  "tokio-macros",
  "windows-sys 0.61.2",
 ]