This commit adds the _xpack/usage api to the high level rest client.
Currently in the transport api, the usage data is exposed in a limited
fashion, at most giving one level of helper methods for the inner keys
of data, but then exposing thos subobjects as maps of objects. Rather
than making parsers for every set of usage data from each feature, this
PR exposes the entire set of usage data as a map of maps.
Ryan Ernst