We would like to select `lightwalletd` servers with mobile and downstream-of-zingolib clients

[dannasessha]

Right now zingo-cli allows for a selection of which lightwalletd server it will use as an intermediate for interacting with zcash as a whole (generally using zcashd).

Clients who depend on the library aspect of this repo should be able to easily select a server as well.

This issue is intended to begin tracking the issue, potentially by exposing some functionality (via API?) within the lib, and also to serve as a starting point to open issues in other repos to connect to that functionality.