Add Unitree Z1 with gripper model

[louisgevers]

This adds an extra Z1 model z1_gripper.xml that includes the compatible Z1 gripper, extracted from the URDF using the same steps for creating the existing z1.xml:

[kevinzakka]

@louisgevers The collision mesh for the gripper needs to be tweaked to actually allow for things to be grasped. This is what is currently being computed from the convex hull of the mesh:

We can either use CoACD for this or hand design some primitive geometries. Do you want to take a stab at this?

[louisgevers]

Thanks for catching this; I’ll give it a go and update the PR accordingly