Add generic completion of any task

See: https://docs.camunda.org/manual/7.17/reference/rest/task/post-complete/

As process tester I want to complete _any_ task. At the moment I can only complete external task.

Unfortunately, CamundaLibrary had focuesed on external tasks and named the completion keyword for external tasks `Complete Task` which would be indeed perfect for the functionality required in this issue.

Maybe add an optional argument `task_id` to `complete_task` keyword. If the id is given, call the generic completion endpoint.

Start at the implementation of the existing complete_task keyword and compare its external-task behavior with the generic completion endpoint described in the linked Camunda REST documentation. Done means complete_task still supports external tasks and also completes any task when task_id is supplied.

Written by the indexing model from the issue text.