Async fs support

[Xuanwo]

Background

tokio::fs is just a wrapper of std::fs, we can adopt aio or io_uring to get more throughput in our pure async framework.

Some ideas

• Provide different schemes vs The same scheme fs with the flag or feature flag?
• Check os support during build. (io_uring requires Linux 5.6)
• Inside opendal or another new crate?

Possible solutions

io_uring

• https://github.com/tokio-rs/tokio-uring: require a new runtime.
• https://github.com/spacejam/rio: used in sled, unsound.
• https://github.com/ringbahn/ringbahn: experimental, extra userspace copy
• https://github.com/datenlord/ring-io: early development stage, experimental only

aio

• https://github.com/hmwill/tokio-linux-aio: runtime related.