bullet-featherstone: Non-base links return zero velocities

[iche033]

Environment

• OS Version: Ubuntu 22.04
• Source or binary build? source build, gz-physics7 and main branches

Description

• Expected behavior: A moving non-base link should return correct linear and angular velocity values.
• Actual behavior: Zero velocities are returned

In the bullet-featherstone plugin, non-base links currently returns zero linear and angular linear velocities. The relevant code for generating velocities for non-base links are located here: https://github.com/gazebosim/gz-physics/blob/2bc19ad0d7ca34f2150c765e8cce5d712ba117bd/bullet-featherstone/src/KinematicsFeatures.cc#L43-L44

A closer look at the bullet source code reveals that the btMultiBodyLink's m_absFrameTotVelocity variables are never filled unless btMultiBody::useGlobalVelcoties() is called and set to true