Introduce b1z1_description: B1 quadruped + Z1 robotic arm

[paulblum]

This PR introduces b1z1_description, a model description that combines the Unitree B1 quadruped with the Unitree Z1 robotic arm.

b1z1_description in Gazebo; unitree_servo and z1_controller's demo trajectory visualized here

The controllability of b1z1_description relies on two prior pull requests:

• Reimplement computeTorque() for Accurate B1 Motor Simulation #89
  • Removes a ±55 Nm clamp from B1 joints, enabling full ±140 Nm torque
• Add support for controlling Z1 arm mounted on Aliengo quadruped (aliengoZ1 model) z1_controller#7
  • Enables z1_controller to be used to control the Z1 arm on quadrupeds

Notably, this PR proposes reimplementation of the b1_description and z1_description packages to define models as xacro macros. This approach allows for the creation of the B1+Z1 model with great simplicity. Key benefits include:

• Updates to parent models (B1 or Z1) will automatically propagate to child models (B1+Z1)
• The framework is easily generalizable for creating model descriptions that mount the Z1 robotic arm onto other quadrupeds

Your feedback is welcome, and I would be happy to address any suggested changes.