Adds the SetMimicConstraintFeature with an implementation
in bullet-featherstone using btMultiBodyGearConstraint.
The MimicMotorConstraint in dartsim does not create a
bilateral constraint between joints, so it does not satisfy
the requirements of this feature, so only the
bullet-featherstone implementation is provided for now.

Signed-off-by: Aditya Pande <adityapande@google.com>
Signed-off-by: Steve Peters <scpeters@openrobotics.org>
Co-authored-by: Aditya Pande <adityapande@google.com>