[camunda-bpm-data] Provide reader for historic variable values

[jangalinski]

Scenario

The process instance has ended but I want to access variables of it. Concrete Usecase: verifying in tests.

Current Behaviour

I need to manually do a historyService veriable query and cast the result.

Wanted Behaviour

I can use CamundaBpmData.reader(historyService, processInstanceId).get(myFactory),