Add camera follow entity feature

[Likhithsai2580]

Fixes #538

Add functionality for the camera to follow an entity.

• Camera Class (genesis/vis/camera.py)

  • Add _follow_entity, _follow_height, and _follow_smoothing attributes.
  • Add follow_entity method to set the camera to follow a specified entity.
  • Update render method to call follow_entity if enabled.

• Scene Class (genesis/engine/scene.py)

  • Add set_camera_follow_entity method to set the camera to follow a specified entity.
  • Update step method to call set_camera_follow_entity if enabled.

• Drone Example (examples/drone/interactive_drone.py)

  • Add --follow argument to enable camera follow.
  • Update main function to use follow_entity method if --follow argument is provided.

[CarlosMagni]

I had an error while running the Drone Example with --follow, it says :
gs.raise_exception("Scene is already built.")
[Genesis] [23:33:32] [ERROR] Scene is already built.

Actually it seems logical as you're trying to add a cam to the scene after the build()
However when I tried to move the build function after adding the new cam I got another error

.pyenv/versions/3.10.10/lib/python3.10/site-packages/genesis/engine/scene.py", line 935, in set_camera_follow_entity
camera.follow_entity(entity, height, smoothing)
TypeError: Camera.follow_entity() takes 1 positional argument but 4 were given

Hope this help