bullet-featherstone: support spring stiffness and reference parameters

[scpeters]

Part of #545.

Desired behavior

The //joint/axis/dynamics/spring_stiffness and //joint/axis/dynamics/spring_reference SDFormat parameters are not currently supported by bullet-featherstone, but they should be.

Alternatives considered

Implementation suggestion

This is tricky because while I see joint friction and damping in bullet's Multibody API, I don't see spring stiffness. If implicit stiffness is not in the API, we can use an explicit effort applied at each time-step instead (see ODEJoint::ApplyExplicitStiffnessDamping from gazebo-classic).

Additional context