Add Psr18Client to make it straightforward to use PSR-18

[nicolas-grekas]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets	-
Documentation	-
License	MIT

Psr18Client extends Psr17Client so that this dramatically cuts the boilerplate to make an HTTP request:

use Http\Discovery\Psr18Client;

$client = new Psr18Client();
$request = $client->createRequest('GET', 'https://example.com/');
$response = $client->sendRequest($request);

Internally, this code reuses whatever PSR-7, PSR-17 and PSR-18 implementations are installed.