Related to #709. There are a few things being considered here;

1. Development of the newton schema/attributes; This is the newtonic way of authoring USD. They are attributes that are missing in UsdPhysics, but defined in other engines like physX or mujoco, and Newton can directly use them in model builder, e.g. joint armature or geoms contact margin, etc.

2. When working with physX-based assets, we should still be able to remap the physX values to newton. The same applies to Mujoco. We should be able to make use of the mujoco attributes that are recognized by Newton.

3. The user can define priority for parsing, i.e. define which engine attributes should be preferred.

4. There are attributes that are specific to each engine/solver, and we may not be able to directly use them. They are usually prefixed with defined terms, like physxCollision, physxRigidBody, etc. For mujoco, we can reuse these later on when building the mjModel though. This PR makes these attributes accessible as part of the parsing results.

5. The USD asset can define custom attributes that are expected to become part of the model/state/control attributes. Parsing of these special attributes are made possible in this PR. The custom attributes are expected to follow the newton:assignment:frequency:attribute_name where assignment can be model/state/control/contact and frequency defines whether the attribute_name is a per body/shape/articulation etc.

Description

Newton Migration Guide

Please ensure the migration guide for warp.sim users is up-to-date with the changes made in this PR.

• The migration guide in docs/migration.rst is up-to date

Before your PR is "Ready for review"

• Necessary tests have been added and new examples are tested (see newton/tests/test_examples.py)
• Documentation is up-to-date
• Code passes formatting and linting checks with pre-commit run -a

Summary by CodeRabbit

• New Features

  • Custom attributes: declare per-model/state/control/contact attributes at BODY/SHAPE/JOINT/JOINT_DOF/JOINT_COORD/ARTICULATION frequencies; persisted through builds/merges and exposed on Model/State/Control.
  • Resolver-driven USD import: pluggable schema resolvers with solver-specific attribute extraction, per-prim custom attributes, per-DOF/per-axis joint states/limits (including D6), and propagation of physics_dt, max_solver_iterations, and solver_specific_attrs.

• Public API

  • Added SchemaResolver family and resolver manager; exposed ModelAttributeFrequency and ModelAttributeAssignment enums; ModelBuilder/Model APIs extended to declare, accept, and materialize custom attributes and updated USD import signatures.

• Tests & Assets

  • Added comprehensive resolver and custom-attribute test suites and new USD assets (ant_mixed, humanoid).