Allow the use of stage-name to trigger adhoc fragment

Currently, triggering adhoc fragments in a stage via CaseService requires you to pass the stage ID. That ID is generated by our editors, which makes it pretty hard to, for example, people that write rules that need to dynamically trigger these fragments to point to the correct stage.

jBPM should also support triggering stages by name. This would make it a lot easier to target a specific stage. This would also allow us to trigger ad hoc fragments in parameterised stage names (i.e., stage-names that have a property-placeholder in their name), which would again be really useful in the case of multi-stage scenarios.

My impression is that the fix should be in this line by simply adding:
.filter(stage -> stage.getId().equals(stageId) || stage.getName().equals(stageId))

We might also want to change the variable name and description of the API from stageId to something like stageIdorName to be a bit more clear on semantics.