Create schema for services/integrations

[davidkpiano]

The schema would need to include the following:

• input: The schema for the initial context that is used to initiate the service. This includes things like API keys and other data (required or optional)
• events: The schema for the events that the service can receive. This can be the same as e.g., GET, PUT, POST, etc. requests to RESTful services, or events that affect the local behavior of the service but don't actually reach out to the service (e.g., retrieving cached data)
• output: The schema for the events that are emitted by the service.
• data: The schema for donedata that is emitted as a "done.invoke.[id]" event when the service is terminated.

Prior art:

• https://docs.n8n.io/nodes/creating-nodes/create-node.html
• https://docs.microsoft.com/en-us/connectors/custom-connectors/create-logic-apps-connector
• https://platform.zapier.com/partners/lifecycle-planning