HttpClient: Separate Client from Transports

[adamziel]

Replaces inheritance (AbstractClient > CurlClient, SocketsClient) with a single Client class that uses either a SocketsTransport or CurlTransport:

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

// Transport is auto-selected:
$client = new Client();

// You can also choose a transport explicitly:
$client = new Client([ 'transport' => 'curl' ]);
$client = new Client([ 'transport' => 'sockets' ]);

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

This is a pre-requisite to #18

Testing instructions

Tests are included – confirm the CI checks pass