Make net::test_server::HttpConnection into an interface

This will make extending HttpConnection to HTTP/2 easier.

Highlights include:

 * Remove HttpConnection as a "friend" class of EmbeddedTestServer
    * replacing all references to private members of HttpConnection with
      getters and setters
 * Making HttpConnection into a purely virtual interface
    * Moving the implementation to "Http1Connection"
 * replacing most of the callback passing with a delegate.

Bug: 1232482
Change-Id: Ia0dddc630ae85d4fcd5ac796e9831b240a2ed71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3052557
Commit-Queue: Aaron Tagliaboschi <aarontag@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919954}