idea: Provide an adapter to std::fs and tokio::fs

[Xuanwo]

By providing this adapter, users can use opendal as a drop-in replace of std::fs and tokio::fs.

- use tokio::fs
+ use opendal::adapter::fs;

async fn main() {
   let _ = fs::try_init();

   let f = fs::create("abc").await?;
}

But we need to figure how to init our services.