Support for asynchronous task processing

[stephanpelikan]

In some situations one needs to implement asynchronous task processing. Camunda 7 provides the external task pattern to support this. This pattern should be integrated into the vanillabp-camunda7-adapter according to the VanillaBP definition.

In the past there was another OpenSource project dealing with external tasks: https://github.com/stephanpelikan/camunda-externaltask-handler. This can be taken as a basis for the implementation.