Add a Viewer-to-Sim Snapshot channel and convert relevant messages.

The ViewerEndpoint can now send Snapshot messages to the SimEndpoint. The SimEndpoint has a new method, `get_viewer_snapshots`, to retrieve these. Thsi channel will be used for UI state needed for the simulation (e.g., `MjOption` and `StepControl`).

PiperOrigin-RevId: 940981799
Change-Id: Ib9bd6ad848cd5479bc49492a34fddd82569cb09f

python/mujoco/experimental/studio/messages.py

@@ -113,8 +113,8 @@ class ModelEvent(Event):
 
 
 @dataclasses.dataclass(frozen=True)
-class OptionEvent(Event):
-  """An event that transports updated MuJoCo model options."""
+class MjOptionSnapshot(Snapshot):
+  """A snapshot sending mjOption state from viewer to sim each frame."""
 
   opt: mujoco.MjOption
 
@@ -128,8 +128,8 @@ class PerturbEvent(Event):
 
 
 @dataclasses.dataclass(frozen=True)
-class StepControlEvent(Event):
-  """An event carrying the full step control state from the viewer to the sim."""
+class StepControlSnapshot(Snapshot):
+  """A snapshot sending step control state from viewer to sim each frame."""
 
   pause_state: sim.PauseState
   speed: float