bullet-featherstone feature parity: tracking issue

[scpeters]

The bullet-featherstone plugin has implemented many gz-physics features, but not all. This issue will track which features are yet to be implemented

• bullet-featherstone: support off-diagonal inertia #544
• bullet-featherstone: fix how links are flattened in ConstructSdfModel #562
• bullet-featherstone: fix setting angular velocity #567
• bullet-featherstone: implement ConstructSdfNestedModel feature #546
• bullet-featherstone: implement WorldModelFeature feature #547
• bullet-featherstone: support spring stiffness and reference parameters #549
• bullet-featherstone: support multiple kinematic tree roots in a single model #550
• bullet-featherstone: F/T sensor reports incorrect values at fixed joint if joint forces are applied #565
• bullet-featherstone: implement support for contactSurfaceMotionVelocity #569
• joint_position_controller example world behaves differently in dartsim, bullet* gz-sim#2162