bullet-featherstone: support joint damping and friction parameters

[scpeters]

Part of #545.

Desired behavior

The //joint/axis*/dynamics/damping and //joint/axis*/dynamics/friction parameters are read from SDFormat by the bullet-featherstone SDFFeatures::ConstructSdfModelImpl method and written to the m_jointDamping and m_jointFriction member variables of btMultibodyLink, but a todo code comment in the btMultibodyLink.h indicates that those variables are not yet used.

We either need to implement support for these parameters in upstream bullet or implement an explicit joint damping workaround (see gazebo::physics::ODEJoint::ApplyExplicitStiffnessDamping from gazebo-classic for reference).

Alternatives considered

Implementation suggestion

Additional context