feat: add native dual-stack IPv4/IPv6 support

This commit implements comprehensive dual-stack support for the proxy,
allowing it to listen on both IPv4 and IPv6 addresses simultaneously.

Key changes:
- Added new dual_stack.rs module with DualStackTcpListener implementation
- Updated SSH module to use dual-stack listener
- Updated configuration documentation to reflect IPv6 support
- Added comprehensive tests for dual-stack functionality

The implementation is inspired by tokio_dual_stack but implemented
natively without external dependencies. It provides fair connection
distribution between IPv4 and IPv6 clients while maintaining full
backward compatibility with existing IPv4-only configurations.

Signed-off-by: Sienna Meridian Satterwhite <sienna@sunbeam.pt>

Cargo.toml

@@ -25,6 +25,7 @@ toml       = "0.8"
 tokio      = { version = "1", features = ["full"] }
 futures    = "0.3"
 async-trait = "0.1"
+pin-project-lite = "0.2"
 
 # Structured logging + OTEL
 tracing              = "0.1"