Add a WAF extension

[bianpengyuan]

This is the track the effort to get https://github.com/istio-ecosystem/wasm-extensions/pull/2 in. The remaining work includes:

• Add unit test and integration test
• Use bazel for its build
• Add doc about how to customize the extension

cc @fpesce I will add guide on how to use bazel and add tests coverage to a Wasm extension.