OpenDAL std::fs / tokio::fs Adapter

[Xuanwo]

It could be intereting to provide an adapter for std::fs or tokio::fs so users can migrate to opendal as easy as:

For std::fs:

- use std::fs;
+ use opendal_std_fs;

For tokio::fs:

- use tokio::fs;
+ use opendal_tokio_fs;

There are two main issues we need to address:

• Naming: yes, naming is of course the hardest problem.
• Init: std::fs & tokio::fs are accessing global resources file system. But opendal needs to initiate first. We need to decide how users will use them.

[Xuanwo]

For naming:

• In different crate: opendal_std_fs and opendal_tokio_fs.
• In the same crate: openfal::std_fs and openfal::tokio_fs (openfal == Open File Access Layer)

Do you have other ideas?