Provide `norpa` and `rpa` option on startup

If we have test cases and tasks on execution path, Robot won't start up, even when we only execute either tests or tasks.

We currently must provide robot with the `--rpa` or `--norpa` option until https://github.com/robotframework/robotframework/issues/4360 is fixed.

This issue is closed, when you can provide the option to webservice.

Start by tracing how the webservice starts Robot and how startup options are accepted. Add an exposed way to select --rpa or --norpa, then verify that the service can launch when both tests and tasks exist while executing either mode.

Written by the indexing model from the issue text.