Adding GetDownline to the public API

While the public API has an endpoint to get all of a customer's downline ids, it does not have an endpoint that describes the structure/relationships of the tree. It would be nice to have and endpoint that lists downline ids paired with their upline id and leg info so that consumers can construct the tree.

I believe there is already logic to do this in abstractions exposed in the client DLL to get a customer's downline and it would be great just to see that functionality exposed through the client API.