Add CachedWsdlProvider

[veewee]

Fixes #63

The old HttPlugWsdlProvider does not work exactly the way you would think it works.
This PR is contains an improved CachedWsdlProvider with a separated HttpWsdlLoader
This implementation makes it possible to cache a WSDL temporary or permanent.

Usage:

<?php
use Http\Client\Common\PluginClient;
use Http\Discovery\Psr17FactoryDiscovery;
use Phpro\SoapClient\Wsdl\Loader\HttpWsdlLoader;
use Phpro\SoapClient\Wsdl\Provider\CachedWsdlProvider;
use Symfony\Component\Filesystem\Filesystem;

$loader = new HttpWsdlLoader(
    new PluginClient($httpClient, [
         // You can add WSDL middlewares in here. E.g.: authentication, manipulations, ...
    ]),
    Psr17FactoryDiscovery::findRequestFactory()
);
$provider = new CachedWsdlProvider($loader, new Filesystem(), sys_get_temp_dir());
$wsdl = $provider->provide('https://somehost/service.wsdl');

Another mode is the forcePermanentDownloads mode:

$provider->forcePermanentDownloads()->provide('http:https://somehost/service.wsdl');

This enables you to create a CLI command that caches WSDLs for production environments that don't change often in a cronjob. This means that the WSDL won't be downloaded from the remote server on every request.