Add a new plugin / extension mechanism called a resource provider along with retrofitting VFS on top of it.

A resource provider provides a mechanism for MuJoCo to read from filesystems other than the OS filesystem or the Virtual File System (VFS).

PiperOrigin-RevId: 525394983
Change-Id: I077ff5a7e2e76806b48b6defb531280aadc8b169

src/xml/xml.h

@@ -25,8 +25,8 @@
 // Main writer function
 bool mjWriteXML(mjCModel* model, std::string filename, char* error, int error_sz);
 
-// Main parser function: from file or VFS
-mjCModel* mjParseXML(const char* filename, const mjVFS* vfs, char* error, int error_sz);
+// Main parser function
+mjCModel* mjParseXML(const char* filename, int default_provider, char* error, int error_sz);
 
 
 #endif  // MUJOCO_SRC_XML_XML_H_