feature: expose redirect policy through extension

[unusual-thoughts]

This adds a FollowedPolicy extension for the FollowRedirect middleware that exposes the policy struct to the response.

Motivation

Currently there is no way to inspect the URLs (and status codes) that a redirected response took, as there is no way to "get back" the redirect policy struct from follow_redirect::ResponseFuture.

Custom redirect policies might have useful state that the user wants to access once the request completes. A typical use case would be to log the urls and status codes that have been traversed, which is already necessary for example to avoid loops.

This implements part of the modifications suggested in #404 as I understand it. It is also required to implement seanmonstar/reqwest#2606

Solution

Implement a http::Extensions::Extension that stores a copy of the follow_redirect::policy::Policy struct that was run on the last request of the redirect chain.

Downsides

Currently this does another clone() of the policy struct for each new request

[unusual-thoughts]

Rewrote to avoid breaking changes. Did my best to avoid code duplication, but some is unavoidable.

[unusual-thoughts]

might be useful to make this fully public

[seanmonstar]

Here's another possible option, which I welcome feedback on from others: what if all that was added was a new method Policy::on_response(&mut Response)?

reqwest could then implement that method to insert an extension.

[unusual-thoughts]

Here's another possible option, which I welcome feedback on from others: what if all that was added was a new method Policy::on_response(&mut Response)?

reqwest could then implement that method to insert an extension.

I suppose that could work too.