Extend Manual Complete step to accept our own webhooks in the middle of a Workflow

[I-Dont-Remember]

Hypothetical scenario, send a webhook out - then wait for the response to come back before continuing Workflow.

This adds yet another 🪛 in the 🧰 of people using Workflow Builder.

This can be super cool: Run a workflow, send a webhook to another service (or Workflow!), then wait for a webhook to come back and continue the flow.

[I-Dont-Remember]

This was added in #44