openapi: Fetch protobuf rather than Json
openapi: Fetch protobuf rather than Json

This is much faster.

Kubernetes-commit: 224dba9a136e3e4014d88b3c2546418066bac026

caesarxuchao/sample-apiserver,caesarxuchao/sample-apiserver,caesarxuchao/sample-apiserver,kubernetes/sample-apiserver,kubernetes-nightly/sample-apiserver,kubernetes/sample-apiserver,kubernetes-nightly/sample-apiserver