Copy Validator to email-editor package

Because the email-editor package should be independent. We copy Validator to the package, at least for now.
[MAILPOET-6216]

packages/php/email-editor/src/Validator/Schema/AnyOfSchema.php

@@ -0,0 +1,37 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\EmailEditor\Validator\Schema;
+
+use MailPoet\EmailEditor\Validator\Schema;
+
+// See: https://developer.wordpress.org/rest-api/extending-the-rest-api/schema/#oneof-and-anyof
+class AnyOfSchema extends Schema {
+  protected $schema = [
+    'anyOf' => [],
+  ];
+
+  /** @param Schema[] $schemas */
+  public function __construct(
+    array $schemas
+  ) {
+    foreach ($schemas as $schema) {
+      $this->schema['anyOf'][] = $schema->toArray();
+    }
+  }
+
+  public function nullable(): self {
+    $null = ['type' => 'null'];
+    $anyOf = $this->schema['anyOf'];
+    $value = in_array($null, $anyOf, true) ? $anyOf : array_merge($anyOf, [$null]);
+    return $this->updateSchemaProperty('anyOf', $value);
+  }
+
+  public function nonNullable(): self {
+    $null = ['type' => 'null'];
+    $anyOf = $this->schema['anyOf'];
+    $value = array_filter($anyOf, function ($item) use ($null) {
+      return $item !== $null;
+    });
+    return $this->updateSchemaProperty('anyOf', $value);
+  }
+}