Add Support for WAF

*Description*:

Add WAF support. The implementation should focus on [coraza-proxy-wasm](https://github.com/corazawaf/coraza-proxy-wasm) but allow other WAF solutions in the future.
- [ ] Create a design doc for the user-facing API. See #529 (draft) as an example.
- [ ] Update xds IR.
- [ ] Update the Kube provider to reconcile the WAF Custom Resource.
- [ ] Update the resource translator to translate the WAF Custom Resource into IRs.
- [ ] Update the xds translator to translate the xds IR into xds APIs, e.g. ECDS.
- [ ] Add a user-facing doc and link into Sphinx docs, e.g. `user_docs.rst`.
- [ ] Others?

~This issue depends on #71 and/or #24.~