Add Envoy TCP Routing example demonstrating source-based dynamic cluster selection

[SiiiTschiii]

Introduce a new Proxy-Wasm TCP filter example that demonstrates dynamic routing of TCP connections based on source IP addresses at the TCP stream context (L4). This example showcases application-layer routing decisions without HTTP processing, filling a gap in the existing HTTP-focused examples.

Inspired by the wasmerang project, this example demonstrates advanced TCP routing patterns in Envoy/Istio/K8s environments using WASM filters.

Key Features

• Source-based routing: Dynamic cluster selection based on source IP last octet (even → egress-router1, odd → egress-router2)
• TCP stream context: Operates at L4 rather than HTTP layer for protocol-agnostic routing
• Envoy filter state API: Uses set_envoy_filter_state foreign function with protobuf encoding
• Complete Docker setup: Includes Docker Compose environment for testing with httpbin.org
• Comprehensive documentation: Usage examples, expected output, and technical context

Implementation Details

• Protobuf definitions for Envoy's filter state API with build-time code generation
• MSRV-compatible dependencies (Rust 1.65) with explicit pinning for CI compatibility
• Envoy v1.34.1 configuration with dual upstream clusters

Documentation Highlights

• Links to Proxy-Wasm spec for TCP stream context
• Explanation of TCP vs HTTP layer routing trade-offs
• Future enhancement ideas: PROXY protocol support for source-based egress routing in managed K8s environments

This example is useful for scenarios requiring performance-critical or protocol-agnostic routing decisions where application-layer processing should be avoided.