Build cloud backends separately

TaskChampion itself doesn't have a lot of dependencies, and builds pretty quickly. However, the GCP and AWS cloud SDKs each have _large_ dependency trees, with surprisingly little overlap.

What could we do to de-couple those? Ideally saving on one or more of
- Binary size
- Build time
- Dependency churn (i.e., number of dependabot updates)

I don't have any good ideas for how to do this. Runtime dynamic linking in Rust basically requires FFI, so that's not a great option. And invoking a separate process is difficult for a library - how would that work from Python? from C++?