Move OpenUSD parsing to usd_decoder

Add usd_decoder to mujoco/plugin.
  - Previously was src/experimental/usd/usd_to_mjspec.cc, now that same spec is returned by the plugin.

PiperOrigin-RevId: 854350962
Change-Id: Ia980190a557c50ce8197980922a2594b613859b4

plugin/usd_decoder/material_parsing.h

@@ -0,0 +1,23 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_PLUGIN_USD_DECODER_MATERIAL_PARSING_H_
+#define MUJOCO_PLUGIN_USD_DECODER_MATERIAL_PARSING_H_
+
+#include <pxr/usd/usdShade/material.h>
+#include <mujoco/mujoco.h>
+
+mjsMaterial* ParseMaterial(mjSpec* spec, const pxr::UsdShadeMaterial &material);
+
+#endif  // MUJOCO_PLUGIN_USD_DECODER_MATERIAL_PARSING_H_