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

include/mujoco/mjmodel.h

@@ -369,15 +369,31 @@ typedef struct mjLROpt_ mjLROpt;
 
 //---------------------------------- mjVFS ---------------------------------------------------------
 
-struct mjVFS_ {                   // virtual file system for loading from memory
-  int   nfile;                    // number of files present
+struct mjVFS_ {                            // virtual file system for loading from memory
+  int   nfile;                             // number of files present
   char  filename[mjMAXVFS][mjMAXVFSNAME];  // file name without path
-  int   filesize[mjMAXVFS];       // file size in bytes
-  void* filedata[mjMAXVFS];       // buffer with file data
+  int   filesize[mjMAXVFS];                // file size in bytes
+  void* filedata[mjMAXVFS];                // buffer with file data
 };
 typedef struct mjVFS_ mjVFS;
 
 
+//---------------------------------- mjResource ----------------------------------------------------
+
+struct mjResource_ {
+  char* name;                     // name of resource (filename, etc)
+  void* data;                     // opaque data pointer
+  const void* provider_data;      // opaque resource provider data
+
+  // reading callback from resource provider
+  int (*read)(struct mjResource_* resource, const void** buffer);
+
+  // closing callback from resource provider
+  void (*close)(struct mjResource_* resource);
+};
+typedef struct mjResource_ mjResource;
+
+
 //---------------------------------- mjOption ------------------------------------------------------
 
 struct mjOption_ {                // physics options