Use templates for outgoing webhooks

[squaremo]

Some systems we want to integrate with may expect specific payloads in webhooks. One example is https://docs.github.com/en/rest/reference/repos#create-a-repository-dispatch-event (which also needs credentials -- another requirement!). To make this convenient, a generic webhook target could include in its definition a template for the payload, where the template data is the event that arrived.

Some remarks:

• you are very likely to want structured data on the way out; so possibly a structured template is indicated
• the events are mostly metadata e.g., object name, and this might not be enough for many purposes (e.g., if you want to know the commit that was pushed by automation)