[RFE] Support for Configuring ImportedServiceSets with Label Selectors

I am working with a customer who would like to use ImportedServiceSets with LabelSelectors. 

This feature will help them to adopt Security Team CISO.

The yaml would look as follows: 

kind: ImportedServiceSet
apiVersion: federation.maistra.io/v1
metadata:
  name: red-mesh
  namespace: green-mesh-system
spec:
  importRules:
  - type: LabelSelector
    labelSelector:
      namespace: red-mesh-bookinfo
      selector:
        matchLabels:
          export-service: "true"

As per the customer they have suggested to add of the labels in the ExportServiceSet structure and to do the same in the structure in the ImportServiceSet, that will permit in the CRD to add the capabilities of having the labels to filter on them.

I have verified that only NameSelector is supported at this point. 

$ oc explain importedservicesets.spec.importRules | grep type -A 2
   type    <string> -required-
     Type of rule. Only NameSelector type is supported.