Delegate orchestrator instantiation

[felipemontoya]

There is more than one place in the code where we do:

from openedx_ai_extensions.workflows import orchestrators  # pylint: disable=import-outside-toplevel

orchestrator_name = self.profile.orchestrator_class  # "DirectLLMResponse"
orchestrator_class = getattr(orchestrators, orchestrator_name)
orchestrator = orchestrator_class(workflow=self, user=user, context=running_context)

We should refactor that into a class method of the orchestrator and delegate every instance of this to the new method. This way there is no risk that we have different instantiations of orchestrators in the codebase