[Diagram Editor] Support workflow input help text

[mxgrey]

We support help text for node builders and section builders which helps users understand how to use a node and what to expect from it.

Currently we don't have any equivalent at the workflow level. When someone creates a workflow it's not necessarily obvious what kind of input it needs or what behavior the users should expect from it.

We should add a field to the Diagram struct to describe the workflow, and then a field to provide input examples.

This could be similar to description and example_configs except we would rename example_configs to example_inputs.

We could put this new information into the editor in the Run Workflow panel: