Expose RedirectAttempt and RedirectAction types

[dtolnay]

These are used in the signature of RedirectPolicy::custom. They are not currently exposed in a way that crates depending on reqwest can name them. That means custom must be called with a closure relying on type inference:

client.redirect(RedirectPolicy::custom(|attempt| {
    /* ... */
}));

It is not possible to factor the logic out into a function:

use reqwest::/* ??? */::{RedirectAttempt, RedirectAction};

fn my_policy(attempt: RedirectAttempt) -> RedirectAction {
    /* ... */
}

client.redirect(RedirectPolicy::custom(my_policy))

Let's expose RedirectAttempt and RedirectAction to make it possible to write such a function.