Add a schema for StatefulService/StatefulStore CRDs

[jsravn]

ref: https://kubernetes.io/blog/2019/06/20/crd-structural-schema/

• As CRDs have become v1 stable now in K8s, they will require a schema going forward.
• Having a schema means we can generate API documentation and users know exactly what fields are available in the CR.

This has some further implications to the current CRD structure. We can't use a common config for StatefulStore, for example. It needs to be split up into structured schemas for each type, like a postgresConfig, and so on.