follow_redirect: expose previous and next request methods

[lucab]

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

• the method for the original/previous request
• the method to be performed on the next request upon following a redirection

This allows consumers to write policies based on both previous and next requests methods.

[lucab]

For context:

• This is a re-spin of https://github.com/seanmonstar/reqwest/pull/2430
• The original usecase is described in https://github.com/seanmonstar/reqwest/issues/1777#issuecomment-2363279906