Refactor mjVFS and resource management.

Resource operations (e.g. mju_openResource, mju_readResource, and
mju_closeResource, etc.) are now all handled by a VFS instance. It
is now up to the VFS to determine which provider to use in order to
handle those operations.

This allows us to dynamically add/remove (aka "mount") providers to
a VFS to handle special requests. mj_addFileVFS and mj_addBufferVFS
have been reimplemented as two such use-cases. Moreover, we expose
the mounting behaviour with two new functions: mj_mountVFS and
mj_unmountVFS.

PiperOrigin-RevId: 861550939
Change-Id: I070eb4bcc2982466c8f368f7918005538baa5185

include/mujoco/mjplugin.h

@@ -27,6 +27,7 @@
 struct mjResource_ {
   char* name;                                   // name of resource (filename, etc)
   void* data;                                   // opaque data pointer
+  mjVFS* vfs;                                   // pointer to the VFS
   char timestamp[512];                          // timestamp of the resource
   const struct mjpResourceProvider* provider;   // pointer to the provider
 };
@@ -42,6 +43,12 @@ typedef int (*mjfReadResource)(mjResource* resource, const void** buffer);
 // callback for closing a resource (responsible for freeing any allocated memory)
 typedef void (*mjfCloseResource)(mjResource* resource);
 
+// callback for mounting a resource (provider), returns zero on failure
+typedef int (*mjfMountResource)(mjResource* resource);
+
+// callback for unmounting a resource (provider), returns zero on failure
+typedef int (*mjfUnmountResource)(mjResource* resource);
+
 // callback for checking if the current resource was modified from the time
 // specified by the timestamp
 // returns 0 if the resource's timestamp matches the provided timestamp
@@ -55,6 +62,8 @@ struct mjpResourceProvider {
   mjfOpenResource open;             // opening callback
   mjfReadResource read;             // reading callback
   mjfCloseResource close;           // closing callback
+  mjfMountResource mount;           // mounting callback (optional)
+  mjfUnmountResource unmount;       // unmounting callback (optional)
   mjfResourceModified modified;     // resource modified callback (optional)
   void* data;                       // opaque data pointer (resource invariant)
 };