xds: investigate io.envoyproxy.controlplane:api

Similar to #6016 which was not possible because the latest version of `io.envoyproxy.protoc-gen-validate:pgv-java-stub` in maven is not up to date as what we needed.

`io.envoyproxy.controlplane:api` in maven is more up to date and even includes `com.google.cloudapi` that #7109 needs. However, we don't know how frequently it updates on a regular basis. It may still fall behind our need.

Inspect the Maven artifact io.envoyproxy.controlplane:api and compare its available APIs with the needs described in #7109 and the failed approach in #6016. Confirm whether its update cadence keeps it current enough for this project; done means documenting whether adopting it is viable.

Scritto dal modello di indicizzazione a partire dal testo della issue.