object Velocity class like Pose

[DavidRecasens]

Add Velocity dataclass

Introduce a Velocity dataclass (analogous to Pose) to replace raw tuples for specifying object velocities, with support for both linear and angular components.

Changes

• Add isaaclab_arena/utils/velocity.py with linear_xyz, angular_xyz fields, validation, zero() factory, and to_tensor().
• Rename set_initial_linear_velocity(tuple) to set_initial_velocity(Velocity) in ObjectBase.
• Update set_object_pose in terms/events.py to accept Velocity.
• Update usages in dynamic_balls_environment.py and test_events.py.

Breaking changes

• ObjectBase.set_initial_linear_velocity renamed to set_initial_velocity and now takes a Velocity instead of a tuple.