Support file_get_contents() as a transport

[Roy-Orbison]

When PHP configuration option allow_url_fopen is truthy (the default), it's easy to use it as a transport. It's probably less error-prone than a raw socket, too.

$response_body = file_get_contents(
	"https://api.createsend.com/api/v3.2/${endpoint}.json"
	, false
	, stream_context_create([
		'http' => [
			'method' => 'POST',
			'header' => [
				'Authentication: Basic ' . base64_encode("${api_key}:"),
				'Content-Type: application/json',
			],
			'content' => json_encode($request_data),
			'ignore_errors' => true, # prevents returning boolean false for error http statuses
		],
	])
);
$response_headers = $http_response_header; # https://www.php.net/manual/en/reserved.variables.httpresponseheader.php

Obviously it'd need a little bit more logic to choose HTTP method, be more generic, etc., but the code's a lot simpler than cURL and raw sockets.

[Radon8472]

Adding support for php native streams would be great, that would make this lib working without extra extensions.