Issue #3031367: Introduce "schematic" normalizers

core/lib/Drupal/Core/Serialization/Attribute/JsonSchema.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Core\Serialization\Attribute;
+
+/**
+ * Attribute for methods to express the JSON Schema of its return value.
+ *
+ * This attribute may be repeated to define multiple potential types.
+ */
+#[\Attribute(\Attribute::TARGET_METHOD | \Attribute::IS_REPEATABLE)]
+class JsonSchema {
+
+  /**
+   * Constructor.
+   *
+   * @param array $schema
+   *   Schema.
+   */
+  public function __construct(
+    public readonly array $schema = [],
+  ) {
+  }
+
+  /**
+   * Get a JSON Schema type definition array.
+   *
+   * @return array
+   *   Type definition.
+   */
+  public function getJsonSchema(): array {
+    return $this->schema;
+  }
+
+}