Would it make sense to make retry `transient?` function public?

[jozuas]

I'm thinking that it would be nice to have the ability to define your own retry function, and in the retry function if the conditions that I care about don't match, default to https://github.com/wojtekmach/req/blob/85bd7437488eeaf013659d1bebdce0511333d4ae/lib/req/steps.ex#L2292

I'll be happy to make a PR if this sounds reasonable.