Support webhooks when creating predictions

[erbridge]

Resolves #17

[zeke]

I can see the thinking behind introducing a webhook object for tidiness, but we might be painting ourselves into a corner here, in that we'd be deviating from the OpenAPI specification. If users were to create a prediction using a generated method that maps to the create a prediction REST API operation, I would expect it to look something more like this:

{
  input: {}
  webhook: `example.com`
  webhookEventsFilter: ['output', 'completed']
}

[zeke]

Love the changes. Looks good!