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

[Xuanwo]

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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.