New cache prewarm API

A new API has been added for services to tag themselves as prewarmable.

To do this, add the cache_prewarmable service tag, and implement PreWarmableInterface.

Example

    tags:
      - { name: cache_prewarmable }

Plugin managers may also use PreWarmablePluginManagerTrait and several core plugin managers have been adapted to use it.

+  /**
+   * Implements \Drupal\Core\PreWarm\PreWarmableInterface.
+   */
+  public function preWarm(): void {
+    $this->getDefinitions();
+  }

Cache prewarming should not depend on request-specific or request-derived information like the current route, language, or theme, although you could specify these explicitly in a preWarm method as long as persistent and any static caching is accounted for. This is because the context in which a cache can be prewarmed could be prior to routing or via the CLI.