Thanks for creating ky! I used it extensively in my libraries and projects.

I'm running into an issue where I'm making a lot of calls in parallel to an API with a rate limit and the 429 retries are all happening at the same time and triggering the rate limit again. I've tested with a forked version of Ky and adding a random jitter to the retry time makes things much better.

I'm happy to contribute a PR, but wanted to see which, if any, solutions you are open to:

1. Add a random jitter (and potentially a way to control it) to the retry calculation here
2. Allow passing a custom function to replace BACKOFF_FACTOR * (2 ** (this._retryCount - 1)) * 1000
3. Make it possible to do this from the beforeRetry hook?

For more context, what I tested was calculating a jitter factor like JITTER_FACTOR * randomBetween(1, -1) and then multiplying the existing retry time by that. It could be configured so that the default value is 1, so there is no change to existing retry time calculation.