Support calling resource methods #354

It is almost possible to call resource methods manually, as in this program which calls the "getKubeconfig" method on cluster:

variables:
  kubeconfig:
    Fn::Invoke:
      Function: google-native:container/v1:Cluster/getKubeconfig
      Arguments:
        __self__: ${someClusterResource}

However, this generates an Invoke, not a Call RPC. The syntax also leaves something to be desired, with the __self__ parameter being an implementation detail that other Pulumi languages do not expose to users.