Deprecation notice: HTTPAPISpec

Hello,

We are planning to remove the support for HTTPAPISpec and HTTPAPISpecBinding (see https://istio.io/docs/reference/config/policy-and-telemetry/istio.mixer.v1.config.client/#HTTPAPISpec) in the upcoming releases.
Per usual guideline, we will keep supporting it with a flag for one release.
Please reach out if you have questions.