feat(client): add capture_connection

[rcoh]

Add capture_connection functionality. This allows callers to retrieve the Connected struct of the connection that was used internally by Hyper. This is in service of #2605.

This pairs with a coming PR that adds a poison API to Connected — the pair of these APIs makes it possible to avoid reusing connections that the client deems to be bad.

Example usage:

use hyper::client::connect::{capture_connection, CaptureConnection};
let mut request = http::Request::builder()
   .uri("http://foo.com")
   .body(())
   .unwrap();

let captured_connection = capture_connection(&mut request);

let client = hyper::Client::new();
// success or failure, it will still work fine.
let _ = client.request(request).await;

let connection_info = capture_connection.connection_metadata();

Although this uses http::Extensions under the hood, the API exposed explicitly hides that detail.

[LucioFranco]

Extension trait?

[rcoh]

discussed offline, will defer for now but could add later

[LucioFranco]

Does this need to live in this file or can we move it out?

[rcoh]

can definitely be moved—any preference for final location?

[seanmonstar]

Looks real clear to me, love it!