Execute: work out how to add an input

[josephjclark]

When running the Execute command, after selecting the workflow to execute, we need to prompt for an input object.

Initially we'll have three options:

1. No input
2. [Previous input]
3. Select File...

Select File opens a file picker

Later, we can consider:

• Suggesting and .json file (apart from workflows) found in the project
• Tracking input files, with a name, in the extension (we'd add them to openfn rc)

Once I've run a workflow with an input, I'd like the extension to remember that input and present it in the dialog