Switch Academy ticking from FTickableGameObject to physics-driven actor

[StephaneCapponi]

The pull request include the following changes:

• Removed FTickableGameObject inheritance from UAcademy.
• Introduced AAcademyStepper actor to drive EnvironmentStep() on the physics tick (similar to Unity's AcademyFixedUpdateStepper).
• Added runtime creation and cleanup of the stepper when Academy initializes and disposes.
• Marked AAcademyStepper as transient so it is not saved into levels or assets.
• Updated documentation comments across Academy and Stepper to clarify their lifecycle and purpose.

This change aligns UnrealMLAgents ticking behavior with Unity ML-Agents, ensuring Academy steps run in sync with physics updates rather than frame ticks.