HttpClient: Separate Client from Transports

[adamziel]

Pre-requisite to #18 (on hold)

Drops client subclasses (AbstractClient > CurlClient, SocketsClient) in favor of transports. With this PR, there's only a single Client class that uses either CurlTransport or SocketsTransport internally.

Usage:

<?php
use WordPress\HttpClient\Request;
use WordPress\HttpClient\Client;

$client = new Client([
	// transport is auto-selected to the fastest one available, but you can
	// also choose one with:
	// 'transport' => 'curl',
	// or
	// 'transport' => 'sockets',
]);

$client->enqueue( new Request('https://w.org/') );
while($client->await_next_event([$request])) {

}