feat: Add embassy-based sleeper for no_std

[wackazong]

No description provided.

[wackazong]

How do you propose to use the EmbassySleeper? I thought the usage of

backon = { version = "x.z", default-features = false, features = ["embassy-sleep"] }

would automatically activate embassy-sleep for the async and the blocking sleeper. That means if the feature embassy-sleep is active, the DefaultSleeper and the DefaultBlockingSleeper should be the embassy sleepers as far as I can see.

Let me know what you think.

[wackazong]

Maybe this is a possibility?

[Xuanwo]

How do you propose to use the EmbassySleeper?

I expect users to use:

fut.retry(builder).sleeper(EmbassyTimerSleeper).await

[wackazong]

I think this is what you want, right?

[Xuanwo]

Thank you @wackazong for building this, really appricated!