Validator extender (#2102)

Added validator extender, integration tests, and deprecated related Validating event

src/Extend/Validator.php

@@ -0,0 +1,58 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Extend;
+
+use Flarum\Extension\Extension;
+use Illuminate\Contracts\Container\Container;
+
+class Validator implements ExtenderInterface
+{
+    private $configurationCallbacks = [];
+    private $validator;
+
+    /**
+     * @param string $validatorClass: The ::class attribute of the validator you are modifying.
+     *                                The validator should inherit from \Flarum\Foundation\AbstractValidator.
+     */
+    public function __construct($validatorClass)
+    {
+        $this->validator = $validatorClass;
+    }
+
+    /**
+     * Configure the validator. This is often used to adjust validation rules, but can be
+     * used to make other changes to the validator as well.
+     *
+     * @param callable $callable
+     *
+     * The callable can be a closure or invokable class, and should accept:
+     * - \Flarum\Foundation\AbstractValidator $flarumValidator: The Flarum validator wrapper
+     * - \Illuminate\Validation\Validator $validator: The Laravel validator instance
+     */
+    public function configure($callback)
+    {
+        $this->configurationCallbacks[] = $callback;
+
+        return $this;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        $container->resolving($this->validator, function ($validator, $container) {
+            foreach ($this->configurationCallbacks as $callback) {
+                if (is_string($callback)) {
+                    $callback = $container->make($callback);
+                }
+
+                $validator->addConfiguration($callback);
+            }
+        });
+    }
+}