Allow varying UrlHelper for path-segregated middleware

[weierophinney]

When creating path-segregated middleware, you may want to provide middleware-specific routing. Once you do that, however, you need a new UrlHelper instance that contains that new router, and which knows the base path to prepend, if you want to generate route-specific URIs.

This patch is the first of several that accommodates this scenario.

The approach it uses is to allow optional constructor arguments to each of the UrlHelperFactory and UrlHelperMiddlewareFactory. Usage is as follows:

// in a ConfigProvider or config file:
namespace Api;

use Zend\Expressive\Helper\UrlHelperFactory;
use Zend\Expressive\Helper\UrlHelperMiddlewareFactory;
use Zend\Expressive\Router\FastRouteRouter;

return [
    'dependencies' => [
        'factories' => [
            Router::class => FastRouteRouter::class,
            UrlHelper::class => new UrlHelperFactory('/api', Router::class),
            UrlHelperMiddleware::class => new UrlHelperMiddlewareFactory(UrlHelper::class),
        ],
    ],
];

In the above, Router:class, UrlHelper::class, and UrlHelperMiddleware::class are virtual services, resolving to the Api namespace. When creating our middleware pipeline, we can now reference the UrlHelperMiddleware service:

$app->pipe('/api', [
    Api\UrlHelperMiddleware::class,
    // ...
]);

Each factory now also implements __set_state(), to ensure that configuration caching will work correctly (the default configuration caching uses var_export(), which requires that __set_state() be defined when include is called).