Implementation options:

• Either include a processInstanceIdIn filter in https://docs.camunda.org/manual/7.11/reference/rest/process-definition/get-activity-statistics/ (con: the backend query is rather complex already)
• Include a reference to an incident in the result for https://docs.camunda.org/manual/7.11/reference/rest/process-instance/get-activity-instances/ (pro: frontend doesn't have to make a separate request; we know which incident belongs to which activity instance, which could come in handy in the future; con: this will require an extra incident query in the corresponding command; we should make sure we fetch all incidents in one query and not in one query per execution)