Add HTTP backend

gRPC is nice, but is awkward to deploy in some situations (using eventlet, running on musl libc, and other problems with the C library). The HTTP proxy API appears to be in a better state these days, so a backend could be created to only communicate using that.

This would expose the same python API.

Start by reading the existing gRPC backend and the HTTP proxy API referenced in the issue. Map how the current client exposes its Python API, then determine the scope needed for an HTTP-only backend. Done means users can access the same Python API without relying on gRPC.

Written by the indexing model from the issue text.