[OPT-3171] Allow to distribute by assignee/candidate group for user task reports

AT:

The new configuration field is only available for user tasks that are grouped by flow node
when none is selected, the user task report stays it is right now
the allowed visualizations for those distributed report are bar and table
when userTask is selected, it will split up the count for each user and each user task, so that the result will either be a grouped bar chart or a hierarchical table
if a report was a pie/area chart before the configuration was switched to userTask, then the visualization is automatically changed to bar chart

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