Support for GSI for geohash queries

[mdesanti]

I'm building an application that stores the latest position for a vehicle. The vehicle ID will always stay the same but its position will change as the vehicle moves.

With the library as is, it is quite complicated to implement. The solution I figured out was using a GSI to store the vehicle ID, querying the record to get the partition key and then update the record. However, this requires a new element to be created since the partition key and range key will be different (different position, hence different partition and sort key) and deleting the old record.

It would be a great feature if the user can decide whether to use the library as is or to store the location information in a GSI. In my case it would mean that I query over the GSI and manage updates through the partition key and range key that I decide to use.