Add FollowRedirect middleware

[tesaguri]

This is a middleware that retries requests to follow HTTP redirections.

This includes Policy trait to customize behavior of the middleware, which is heavily inspired by reqwest::redirect::Policy.

Policy also has an optional method clone_body that tries to clone a request body, like tower::retry::Policy::clone_request. But even if the body cannot be cloned, FollowRedirect uses <ReqBody as Default>::default when <ReqBody as http_body::Body>::size_hint returns zero, so that it can handle common GET requests without clone_body.

The API proposed here is a proof of concept to some extent, and it is fine if this is not fully merged at once (2fe3fea...22212ba 3f8deda...22212ba 3f8deda...0e6a949 is optional in particular).

Fixes #78.

[davidpdrsn]

Really great work on this!

I left some comments but overall I think this setup is nice.

I would also like to hear what @hawkw thinks. Adding her as reviewer.

[davidpdrsn]

I think it looks good! I'm gonna try and integrate it with our code at embark as the final test.

[davidpdrsn]

Its working quite well! I ended up with something like this:

// automatically follow redirects
.option_layer(self.max_redirects.map(|limit| {
    FollowRedirectLayer::with_policy(
        Limited::new(limit).and::<_, _, HttpError>(clone_body_fn(Body::try_clone)),
    )
}))

[lpil]

Hey really cool!

[davidpdrsn]

Think I'm just going to merge this. We can always improve things later.