Convert Stage to a Protocol

The Stage is now implemented as a runtime-checkable Protocol.

Part of this led to the realisation that we can't create a DirectThingClient
based on a Protocol, only on a concrete Thing. While it might be
a good idea to address that in LabThings in due course, for now
we work around it by creating both a Protocol and a stub class. The stub class implements the protocol, and
can be used to declare a dependency.

src/openflexure_microscope_server/things/camera/simulation.py

@@ -25,7 +25,7 @@ from labthings_fastapi.types.numpy import NDArray
 from labthings_fastapi.server import ThingServer
 
 from . import BaseCamera, JPEGBlob
-from ..stage import Stage
+from ..stage import StageProtocol as Stage
 
 
 class SimulatedCamera(BaseCamera):