Proxy-Wasm tests

WebAssembly tests are built using Proxy-Wasm C++ SDK and Proxy-Wasm Rust SDK, as such, they bring their own set of dependencies.

Cargo dependencies.

In order to update Cargo dependencies, please make sure that Rust and Cargo are installed, and run this tool:

bash tools/update_crates.sh

which will regenerate Bazel rules in bazel/external/cargo/.