Use with arbitrary file descriptor

Hi, I am one of the maintainers of [aya](https://github.com/aya-rs/aya) a toolkit for interacting with eBPF programs from Rust. Many eBPF objects are backed by file descriptors, and some of them provide readiness notifications. See [our `RingBuf` tests](https://github.com/aya-rs/aya/blob/61376c4608ebd2c10f05f3e917833832158d7c87/test/integration-test/src/tests/ring_buf.rs) which exercise this via raw epoll as well as with tokio (using [`tokio::io::unix::AsyncFd`](https://docs.rs/tokio/latest/tokio/io/unix/struct.AsyncFd.html)). Does monoio offer something akin to `AsyncFd`? How can I wrap a `RingBuf` so it can be used with monoio?