feat: Implement a macro like #[backon] to retry a give async function

[Xuanwo]

backon is great for retrying a single async function, but it's a bit clumsy to retry 100 functions. Perhaps we could provide a macro so users can add #[backon] to their own async functions instead of repeatedly calling retry.

Implementing macro is quite heavy; I expect it to reside in a separate crate.