Add `webhook_reponse` in `StepCompletePayload`

[midigofrank]

Implementation notes

• The worker reads the event.state object of step:complete in src/events/step-complete
• It looks for a webhookResponse
• If it IS set, update the step:complete payload we send to lightning to include webhook_response

For more info, look at: OpenFn/lightning#4666 (comment)

Tests

[midigofrank]

@josephjclark @doc-han I would appreciate some help in speccing this out, especially this point:

If it finds a key called webhookResponse, it extracts that object and writes it to context.state.webhookResponse (context.state is internal worker state for the run, it is not user state returned by steps. Confusing I know!)