Extend debusine:task-configuration to cover parameters outside of task_data

In multiple cases, we felt the need to be able to rely on the task-configuration mechanism to change work request parameters that are outside of task_data:

• workflow_data.allow_failure: #1077 discussed the idea to make some QA checks non fatal for specific packages
• priority_base / priority_adjustment: #1014 discussed the possibility to change the priority of work requests with task configuration