Associate an output parameter of the underlying process with a workflow
parameter. The workflow parameter (given in the `id` field) be may be used
as a `source` to connect with input parameters of other workflow steps, or
with an output parameter of the process.

A unique identifier for this workflow output parameter. This is
the identifier to use in the `source` field of `WorkflowStepInput`
to connect the output value to downstream parameters.
