Idea to support execution listeners

[ingorichtsmeier]

• Add additional headers to the Camunda 8 service task with class names for the listener implementation
• Invoke the start listeners before the delegate implementation
• Invoke the end listeners after the delegate implementation

Assumption: listeners have to be "clean" as well, e.g. they should not invoke execution.getProcessEngineServices().

The transaction behavior is handled in the task worker as all methods are called in a given sequence and any exception is catched in the adapter.

[jonathanlukas]

I like this idea. We could add this behaviour to our converter as well as to the adapter.

[jonathanlukas]

Scope: Only the migration tool will support this.

[jonathanlukas]

@jangalinski tagging you here to match your PR from the hackday with the issue

[jangalinski]

Thanks. I will do some cleaning up first

[jangalinski]

see #280