I want to use HTTP proxy to access cloudbuild.

I believe it can be done in the way described in GoogleNetHttpTransport#newTrustedTransport javadoc.

• https://developers.google.com/api-client-library/java/google-api-java-client/reference/1.20.0/com/google/api/client/googleapis/javanet/GoogleNetHttpTransport#newTrustedTransport()
• https://github.com/jenkinsci/google-cloudbuild-plugin/blob/google-cloudbuild-0.2/src/main/java/com/google/jenkins/plugins/cloudbuild/client/ClientFactory.java#L78