Support testing User Tasks

[AlexanderPruss]

It is currently prohibitively difficult to test code that uses the Tasklist API to query and compute user tasks. Currently, the only way to get access to user tasks with the In-Memory Zeebe client is to activate Jobs of type UserTask, but this activation is not idempotent and does not contain the information that queries to the TaskList API would.

Goal: Create a concept that allows user tasks to be claimed and completed using spring zeebe test tools.

Suggestion: Add helper functions for this end to Spring Zeebe.