Include geo data for geo/geo id discovery and to support geo_value "*"?

[nmdefries]

Geo helper functions let users easily fetch all geo ids of a given type. This could be extended to our non-US data sources, too, to make those easier to work with.

A wrapper around a geo helper function also lets us easily convert a request for "*" geo values to an explicit list, for the endpoints that don't support the wildcard "*". Related to #235