feat(engine, engine-rest) provide job querying via the activityId

Adds the ability to query jobs depending on the activityId for the Job.
Add support for REST as well.

Related to the [JIRA issue](https://app.camunda.com/jira/browse/CAM-3085) and the [forum discussion](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!topic/camunda-bpm-dev/HugUZxqwJF8)

I am not sure what happens with the formatting, I use the provided formatter and in my eclipse everything looks aligned.