Feature: Add library version metadata to client instantiation

[andrewsg]

NDB should instantiate the underlying client with metadata identifying its version, like the underlying Datastore library: https://github.com/googleapis/google-cloud-python/blob/master/datastore/google/cloud/datastore/client.py#L42

We should include both a new key denoting the ndb version, and also continue to carry over the underlying Datastore library version as well.