[#3479231] First pass at updating plugin_type_example module to use PHP...

[#3479231] Update plugin_type_example module to use PHP Attributes for plugin discovery.

Closes #3479231

modules/plugin_type_example/src/Attribute/Sandwich.php

+<?php
+
+namespace Drupal\plugin_type_example\Attribute;
+
+use Drupal\Component\Plugin\Attribute\Plugin;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Defines a Sandwich attribute class.
+ *
+ * Provides an example of how to define a new attribute type for use in
+ * defining a plugin type. Demonstrates documenting the arguments for the class
+ * constructor to help developers understand how to configure plugin instances.
+ *
+ * When defining attributes for a plugin type start by extending the existing
+ * Drupal\Component\Plugin\Attribute\Plugin class. Then create a __construct()
+ * method whose arguments are the configuration options for the plugin type.
+ *
+ * Classes that extend the Plugin class should always accept $id, $deriver
+ * arguments. Developers can add any additional required, or optional, arguments
+ * necessary to configure their specific plugin type.
+ *
+ * The #[\Attribute(\Attribute::TARGET_CLASS)] line says that the class, Block,
+ * defines a #[Block()] attribute, and that the block attribute can only be used
+ * on classes. Or put another way, the code that the #[Block()] attribute is
+ * annotating must be a class, or it won’t validate.
+ *
+ * @see \Drupal\plugin_type_example\SandwichPluginManager
+ * @see plugin_api
+ */
+#[\Attribute(\Attribute::TARGET_CLASS)]
+class Sandwich extends Plugin {
+
+  /**
+   * Construct a sandwich attribute.
+   *
+   * @param string $id
+   *   The plugin ID.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup|null $description
+   *   (optional) Description of the sandwich.
+   * @param float|null $calories
+   *   (optional) Number of calories in the sandwich.
+   * @param class-string|null $deriver
+   *   (optional) The deriver class.
+   */
+  public function __construct(
+    public readonly string $id,
+    public readonly ?TranslatableMarkup $description = NULL,
+    public readonly ?float $calories = NULL,
+    public readonly ?string $deriver = NULL,
+  ) {}
+
+}