Allow configuring an horizontal pod autoscaler for goflow when it is configured as a Deployment.

(We need to see how it can be done wrt the "make manifest" command and openapi spec generation, maybe this needs to stay without a spec / openapi ; see also comments from mazz here https://docs.google.com/document/d/1TTgywlUn52faDgRPbCbxu_1Wpkmraa92zyWpLC0LN9A/edit#)