Context:
Back-end ticket for OPT-3085.

AT:

• I can evaluate reports where I can aggregate variable values for a single variable across all process instances of the current filter
• the evaluation is also possible across several definition and/or tenant versions
• the aggregation can be of the following types: SUM, AVG, MIN, MAX, MEDIAN
• the aggregation works only for numeric variable types. All other variable types throw an error during the evaluation
• it's possible to combine the new variable aggregation reports
• the report rest-API is adjusted according to the new changes