The workflow an app run of the app version runs ("Try this app"): an
input per input port of the version's app.yml (all optional when there
are several; "input" for an app that declares none), the app's step,
and one output, "results", taking everything the step writes to a
download. Returned as a workflow definition (with the step's
paramSchema and resolvedParams) so a client configures the run like a
workflow's, then POSTs /runs with app instead of a workflowId.