Add a `http.Client` compatible client

[escamoteur]

With the introduction of the cupertino_http and cronet_http only Windows and Linux Dart and Flutter apps can't use the benefits of HTTP2 while staying compatible with he abstract http.Client interface.
So it would be nice if this was added to this package.

[brianquinlan]

Maybe it would make sense for someone to update package:flutter_curl (or create a new curl-based package) to support the package:http Client interface?

Or use package:rhttp.

[escamoteur]

Why? isn't the http2 package no longer maintained?

[brianquinlan]

I replied to this 7 months ago but I suspect that either I read "HTTP2" as "HTTP3" or I was thinking that people would want the benefits of HTTP3 (which package:http2 does not have).

[escamoteur]

Sorry, I missed that reply :-) I guess my main point was that we should make sure that Dart devs on windows are not stuck on Http1 from dart:io and should be able to write Portable code that's why I asked for an implementation of the portable Client interface

[demolaf]

We built exactly this for firebase/firebase-admin-dart#297 (Firestore under high concurrency was hitting dart:io HttpClient's one-connection-per-request HTTP/1.1 limit) and landed it downstream in firebase-admin-dart#305. A pooled Http2Client (BaseClient) backed by a generic ClientPool<T>, multi-host by design, dialing per host:port.

Opened as PR #1956.