Add support for Google Cloud Managed Prometheus

[psikala]

This PR adds support for Google Cloud Managed Prometheus. Functionality is the same as when using prometheus-operator. These two can co-exists at the same time, so migration from GMP <> prometheus-operator should be smooth as possible. I created new CRD; ManagedPrometheusServiceLevel, so no need to do ugly hacking since prometheus-operator and Google Managed are different kind of animals, even the API's are almost the same.