Create mjpDecoder, mj_parse, and mju_decodeResource

- plugin system similar to mjpResourceProvider, but instead of loading a resource it converts an existing mjResource into an mjSpec.
  - The returned spec is then composed into the referencing spec.
  - This enables different file types to generate arbitrary specs, and allows us to separate format parsing from compilation code.

Follow up CLs will move some of the logic in src/engine for PNG, USD, KTX, OBJ loading into decoders.

The mj_parse function MjSpec from a given file, it's a more generic version of mj_parseXML.

In it's implementation, mj_parse as opposed to mj_parseXML will look for any registered decoder and
not assume we are striclty dealing with MJCF.

PiperOrigin-RevId: 826149497
Change-Id: I0ece26904280cb94bd5ded6dd5a565c539d60254

src/xml/xml_native_reader.cc

@@ -3404,25 +3404,24 @@ void mjXReader::Asset(XMLElement* section, const mjVFS* vfs) {
 
     // model sub-element
     else if (name == "model") {
-      string content_type;
-      if (!ReadAttrTxt(elem, "content_type", content_type)) {
-        content_type = "text/xml";
-      }
+      std::string content_type;
+      ReadAttrTxt(elem, "content_type", content_type);
 
       // parse the child
       mjSpec* child = nullptr;
       std::array<char, 1024> error;
       auto filename = modelfiledir_ + ReadAttrFile(elem, "file", vfs).value();
 
-      if (content_type == "text/xml") {
-        child = mj_parseXML(filename.c_str(), vfs, error.data(), error.size());
 #ifdef mjUSEUSD
-      } else if (content_type == "text/usd") {
+      if (content_type == "text/usd") {
         child = mj_parseUSD(filename.c_str(), vfs, error.data(), error.size());
-#endif  // mjUSEUSD
       } else {
-        throw mjXError(elem, "unsupported content_type: %s", content_type.c_str());
+#endif  // mjUSEUSD
+        child = mj_parse(filename.c_str(), content_type.c_str(), vfs,
+                               error.data(), error.size());
+#ifdef mjUSEUSD
       }
+#endif  // mjUSEUSD
 
       if (!child) {
         throw mjXError(elem, "could not parse model file with error: %s", error.data());