Provide bindings that allow workflows to be executed from inside a Python script

[mxgrey]

We're currently working on allowing users to run Python scripts as node operations, but with pyo3 bindings it should be feasible to do the reverse as well: Have a Python script run a crossflow executor and send requests to run workflows.

One way this could work is to providing Python bindings for the DiagramElementRegistry so users can populate the registry with Python-based node builders, perhaps even section builders. Then they could pass in a JSON diagram to build into a workflow, and then send requests for that workflow.

With this it would be possible for the main application logic to be written in Python, and then crossflow can be used by the Python application to execute workflow requests as needed.

[ArizmendiWan]

I opened #190 as an initial step toward this.

The current approach is to expose a small Python-facing Executor that can:

• register Python callback nodes
• inspect metadata for the registered catalog
• run JSON-defined workflows from Python

Right now I use Executor as the surface as I think this is a better first step to get things run in Python, but for future iterations adding bindings for DiagramElementRegistry will make it more configurable.