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:

• 89

  • Removes a ±55 Nm clamp from B1 joints, enabling full ±140 Nm torque
• https://github.com/unitreerobotics/z1_controller/pull/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.