client/redirect: expose previous and next request methods

[lucab]

This augments redirect::Attempt in order to expose two relevant HTTP methods during redirects:

• the methods for the previous request.
• the method to be performed on the next request upon following a redirection.

The two methods can different, notably when redirecting POST requests:
https://en.wikipedia.org/wiki/Post/Redirect/Get

Towards #1777 (comment).

[seanmonstar]

This is certainly an interesting idea! I'm not sure which way I lean here, I suppose this could be a way to mutate the next method?

I'd more quickly merge a fix to only change the method from POST, as the spec seems to outline.

[lucab]

@seanmonstar oh sorry, I see now that my PR description is probably too confusing.
This is only meant to cover the comment from @konstin at #1777 (comment) in order to provide a way to stop following such redirects. The "previous method" is required for such a decision (at least), and I thought that adding the "next method" could be useful too (but I can drop that if you prefer, it can be re-computed from the response code). This doesn't need to be a mutating hook, the current follow/stop policy is enough.

This is not going to close the whole #1777, only tackling one specific follow-up comment.
I don't have any urgent need to fix the rest of the ticket, but if you are ok with following the fetch spec I can try to find some time to assemble a PR to align reqwest with that.