Add WASM agent support for JavaScript runtimes

Compile the agent to WebAssembly for deployment in JS runtimes (Bun,
Node.js, or any runtime implementing the TCPDial/UDPListen interface).

- Split network and UDP code with js/native build tags
- Extract UDP relay into a platform-agnostic shared layer
- Expose runtime-agnostic TCPDial and UDPListen JS hooks
- Add azure-sdk-for-go fork as submodule for WASM-compatible mmf fallback
- Bump aznet dependency
- Document WASM build and JS interface in README

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "pkg/azure-sdk"]
+	path = pkg/azure-sdk
+	url = https://github.com/Atsika/azure-sdk-for-go.git