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/engine/engine_vfs.h

@@ -40,6 +40,9 @@ MJAPI int mj_deleteFileVFS(mjVFS* vfs, const char* filename);
 // delete all files from VFS
 MJAPI void mj_deleteVFS(mjVFS* vfs);
 
+// registers a VFS resource provider; returns the index of the provider
+MJAPI int mj_registerVfsProvider(const mjVFS* vfs);
+
 #ifdef __cplusplus
 }
 #endif