[Subtask] Implement an in-memory file system for testing and validating the FUSE framework.

[diqiu50]

Describe the subtask

Implement an in-memory filesystem for testing and validating the FUSE framework. You need to implement the PathFilesystem trait and support basic file and directory operations:

• create file
• delete file
• open file
• read file
• write file
• stat file
• create dir
• delete dir
• read dir
• stat dir

Parent issue

#5504