Enhance processing architecture with new components

- Added `RenameProcessor` to facilitate key renaming in observations, improving data handling flexibility.
- Updated `__init__.py` to include `RenameProcessor` in module exports.
- Refactored `NormalizationProcessor` and `ObservationNormalizer` to use `rsplit` for better key handling.
- Introduced comprehensive tests for `NormalizationProcessor` and `RenameProcessor` to ensure functionality and robustness.

src/lerobot/processor/__init__.py

@@ -20,6 +20,7 @@ from .observation_processor import (
     StateProcessor,
 )
 from .pipeline import EnvTransition, ProcessorStep, RobotProcessor
+from .rename_processor import RenameProcessor
 
 __all__ = [
     "RobotProcessor",
@@ -29,4 +30,5 @@ __all__ = [
     "StateProcessor",
     "ObservationProcessor",
     "NormalizationProcessor",
+    "RenameProcessor",
 ]