feat(ext): add ext::on_informational() callback extension

[seanmonstar]

This new function allows attaching a callback to a request, such that when it is sent through a hyper client connection, and any 1xx informational responses are received, they are passed to the callback.

Closes #2565

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This takes the unstable client informational feature (introduced in #2594, tracking issue in #2565), and promotes it to a stable API. The way it works is largely the same: a user can embed a callback in an extension in a request, and if present, it is called with each 1xx response parsed for that request.

As long as it was working in an unstable manor for curl, we didn't really discuss further if it is particularly rusty. I welcome feedback to that effect.

Alternatives

• ResponseFuture::poll_informational() - I'll note that in the tracking issue, it was discussed that an alternative should be to add methods to ResponseFuture instead. Some of the downsides I feel with that design are:
  • It's hard to await for both informational and the final response (though not impossible, and that is the design with some other interfaces, through polling).
  • Anything that converts the ResponseFuture into an impl Future or dyn Future (or async fn) slightly higher up loses the ability to call those methods, or expose that functionality to users. For instance, composition in a tower stack.