Support PlatformCredentialsSet versioning

[katyanna]

apiVersion: zalando.org/v1
kind: StackSet
metadata:
  name: pcs
spec:
  {...}
  stackTemplate:
    version: "dev-1"
    spec:
      configurationResources:
        - platformCredentialsSet:
            name: my-credentials
            spec:
              application: pcs-app
              tokens:
                sample-token:
                  privileges:
                  - com.zalando::foobar.write

From this definition, the Stackset controller creates a PlatformCrendentialsSet named pcs-dev-1-my-credentials owned by the Stack.