Event notifications

[opqdonut]

• configurable list of urls to send event notifications to
• use HTTP PUT
• use current event schema
• filtering by event type
• configurable timeout
• configurable headers (e.g. Authorization)
• integration test that mimics external id allocation (see Support for external register numbers #1772)
• add event ids to event notifications
• think about event notification ordering guarantees
• document via swagger
  • callbacks are an OpenApi 3 feature and the clojure tooling is stuck in OpenApi 2 (Support openapi3.0 metosin/ring-swagger#121 Support OpenAPI3 metosin/schema-tools#44)

related to #1772
see also cscfi/rems-deploy#129