Add "imported" field to ImportedServiceSet.status.importedServices

It's currently not completely clear whether a service is imported or not. Consider the following output:

$ oc -n green-mesh-system get importedserviceset/red-mesh -o yaml
...
status:
  importedServices:
  - exportedName: details.red-mesh-bookinfo.svc.green-mesh-exports.local
    localService:
      hostname: ""
      name: ""
      namespace: ""

Since the details service is listed under importedServices, we could assume that it is indeed imported. However, that's not the case. This is indicated by the empty fields under localService.

I suggest we add an explicit imported: true/false field to easily distinguish between services that are merely exported (and ignored) and those that are actually imported into the local mesh.

Also, let's omit the localService field for services that aren't imported.