Have a single task server with multiple backends

[WardLT]

There could be cases where we want to use a different workflow engine for different functions. For example, some functions may be best to run locally (e.g., Parsl's HTEx with LocalProvider) and others should run remotely (e.g., via Globus Compute). It would be great to have a way of separating where each run.

[gpauloski]

I like this. Dependency inject all the things! 🎉