[Feature]: Add starting power offset for the exponential backoff

[nashfive]

What problem are you facing?

Hi guys!
I've noticed that your implementation of the exponential backoff is returning a delay of : 0.5, 1, 2, 4s...

Would it be possible to have a way to provide a custom retry implementation along with the one that you provide? or, as a simpler change, add another parameter to let us decide the initial power parameter? For my use case, I'd need my retries to be performed after 2s, 4s and 8s.
So I was thinking about something like .exponential(base: 2, from: 1) or .exponential(base: 2, skip: 2) ..

What do you think ?

Other Information

No response

[malcommac]

I think it's a nice addition to customize the library; if you want to make a PR I'll be more than happy to look at it and integrate into the next release.