fix: avoid to create operator of memory storage every time

[ZENOTME]

The following assert will fail because the memory storage will create a new operator every time. To maintain the context, we should create the operator at init and clone it later.

   #[tokio::test]
    async fn test_memory_io() {
        let io = FileIOBuilder::new("memory").build().unwrap();
        //let io = create_local_file_io();

        let path = format!(
            "{}/1.txt",
            TempDir::new().unwrap().path().to_str().unwrap()
        );

        let output_file = io
            .new_output(path.clone())
            .unwrap();
        output_file.write("test".into()).await.unwrap();

        assert!(io.is_exist(path.clone()).await.unwrap());
    }

[ZENOTME]

cc @Xuanwo

[Xuanwo]

Thanks, this change makes sense to me.

[Xuanwo]

cc @liurenjie1024 for another look.

[liurenjie1024]

Thanks @ZENOTME for this.