Expose mujoco model compiler settings when loading URDFs

[alberthli]

There are some important settings in URDFs that get loaded into mujoco that must be set by a <mujoco> tag. We should let the user programmatically specify some of the features in the model compiler that may be essential for URDF loading:

• the fusestatic property determines whether the first link of non-free-base robots gets unintentionally absorbed into the mujoco model's worldbody. We should let this be False by default.
• the meshdir property indicates the global or local (relative to the URDF) directory containing the model assets.
• the 'balanceinertia' property detects when inertia matrices are ill-formed and will generate a coarse but physical inertia matrix to replace them so errors aren't thrown.
• the 'discardvisual' property controls whether visual geometries are kept or not (thrown out by default)

To see the full list, see: https://mujoco.readthedocs.io/en/3.0.0/XMLreference.html#compiler