Add Parsl Plugin

[tskluzac]

We should have a plugin to execute a Parsl workflow as part of a larger 'Workflow' activity. This plugin should:
[] use the Parsl configuration/provider on an arbitrary machine (supercomputer, laptop, etc.)
[] have a well-defined model for inputs and outputs.
[] have a mechanism to ensure a Parsl workflow script has proper components.
[] have a logical path for results (i.e., do we have to write to file? Can we send upstream?)

I will write out a plan to create this plugin and gather feedback from the team before building.

Link to Parsl docs: https://parsl.readthedocs.io/en/stable/