add a request_retry_modifier function that can modify a request object before a retry

[iwarshak]

Summary

This PR implements my proposal in #296 (comment).

It introduces a new retry option:

{:delay, milliseconds, request_retry_modifier}

The request_retry_modifier is a function that is invoked before a retry. It receives the current request and returns a modified request.

Why?

Currently, Req does not re-run steps on retries. This new option provides an escape hatch for scenarios where request modification is needed on retry—for example, recomputing a time-based authentication header.

[iwarshak]

Just wanted to update that I used this branch in production for a business critical function where our 3rd party payment API has a rate limit that we hit AND each request has a time dependent authorization signature. This branch allowed me to have the jobs retry and recompute the auth header right before it retries. It worked great for us.

[wojtekmach]

Thanks for following up, this is great to hear. There is a bunch of retry related issues and PRs and I intend to do a full sweep on them eventually, apologies for the delay.