Add support for compio

[George-Miao]

Hi! I'm George Miao, co-author and maintainer of compio. I'm opening this discussion to see if it's feasible to have compio support built into openraft, any suggestions will be helpful. Currently openraft has a wonderful AsyncRuntime trait with tokio being the only and mandatory implementor, and I'd like to add the second. Here are list of things I'd like to do:

• Implement AsyncRuntime for compio, specifically,
  • Oneshot will be provided by futures crate
  • Rng will be rand::rngs::ThreadRng
  • Sleep and Timeout future will be boxed dyn future, since compio uses async fn for most functions
• Seems like Instant trait is only implemented for tokio one. Compio uses std one, so one more implementation needed.