Context:
We decided to handle the external events as an data source as well, so it can be used in event based processes or not, while previously it was assumed to always be part of the event based processes data sources.

AT:

• extend the Event Sources entries of an event based process by a type attribute being either:
  • `camunda`
  • `external`
• a type:`external` event source may only have the `id` & `eventScope` property populated
• the Camunda Event based process API is updated accordingly
• only one entry of type: `external` should be allowed to be present in the event sources list at any time