Support CloudEvents 1.0 Specification

Context:
In October 2019, the Cloud Native Computing Foundation released a specification for describing event data in a common way. CloudEvents seeks to ease event declaration and delivery across services, platforms and beyond! Since we don't want to change the event-based processes API too often this might be an easy pick, to support a public standard that ensures easy handling of our API.

AT:

• the CloudEvents 1.0 specification is implemented
  • this requires the following mandatory properties on the event api
    • id
    • source
    • specversion = "1.0"
    • type (rename former `eventName` property)
  • the following optional properties will be implemented:
    • time (rename former timestamp property + change of format to String encoding: RFC 3339)
  • Event data currently present in `data` will stay as it is, it's in compliance with the spec
  • the following optional properties won't be implemented
    • datacontenttype => we only support json anyway for now
    • dataschema => same reasoning as above
  • the following properties required by us but not reflected in the Cloud events spec need to be defined as Extension Context Attributes:
    • traceId
    • group
      • the existing distributed tracing extension goes into the same direction however the specific format and terminology was considered to complex for our use-case to be a good fit
• the HTTP Protocol Binding is implemented, in particular the Batched Content Mode
• the HTTP Webhook Specification is followed:
  • http method needs to be POST

Out of scope:
encryption of the data block will not be supported