feat: add support for should_retry with arity of 2 to Retry middleware

[mdlkxzmcp]

Closes #578

I also formatted the middleware.ex as it caused the lining test to fail aaaand made a typo in the branch name 🤦

I am open to ideas on how to make this work with what is described in that issue... I needed a way to prevent Retry from repeating POST/PUT requests on timeouts, so I wrote it and at first glance thought it was the same issue, but now I see it isn't :)

Edit: with the addition of the suggested changes by @yordis, this now does in fact close the aforementioned issue 🎊

[yordis]

the best way to document such complexity is by having a @type maybe called should_retry_func and document it there.

You do not have to do it, just commeting so that in the future others could find this comment.

[mdlkxzmcp]

I wanted to do something like that, but I haven't seen anything of the sort in any of the other middlewares... If you want, I can make a follow-up PR that documents all the options. The question is should I leverage typedocs and remove the ## Options section in favor of using something like:

@typedoc "Supported options."
@type options :: %{optional(:delay) => delay(), ...}

@typedoc "The base delay in milliseconds (positive integer, defaults to 50)"
@type delay :: pos_integer()

And then just reference it with t:options/0 in the moduledoc?

[yordis]

I tend to say yes, lets give it a try

[yordis]

@teamon going to wait for your feedback just in case, but thus far, I am OK with the addition.