Remove "riva" from the name. Provide a configuration basis to enable one or more backend drivers instead, including SAPI5, Coqui, and Piper

[kfatehi]

This project does a lot of useful work beyond just connecting to Riva.

Riva is really just a backend.

Let's make this project more generic, and move the Riva client code into a standard interface for a "Remote voice"

This way, we can enable multiple remote voice sets from different providers, e.g. Coqui voices: kfatehi/persian-tts-server#1

Finally, update the ReadAloud project to contextualize this project in the proper way ken107/read-aloud#217

My main goal here though is that people who find the project from ReadAloud should not have an any more confusing or difficult time setting up. Making the architecture more flexible has the potential to damage this experience which is already quite complex. So we need to be thoughtful here.