Allow usage of cacheTags for FPC

Regarding the AbstarctBlock implementation of getCacheTags, tags from identities are merges to cache tags anyway, so we could use it safely.
Also Magento does not recommend to implement new Blocks, so we could declare the block's tags in the layout xml for the FPC.

Author: thomas-kl1

app/code/Magento/PageCache/Model/Layout/LayoutPlugin.php

@@ -10,10 +10,15 @@
 use Magento\Framework\App\MaintenanceMode;
 use Magento\Framework\App\ObjectManager;
 use Magento\Framework\App\ResponseInterface;
+use Magento\Framework\DataObject;
 use Magento\Framework\DataObject\IdentityInterface;
+use Magento\Framework\View\Element\BlockInterface;
 use Magento\Framework\View\Layout;
 use Magento\PageCache\Model\Config;
 use Magento\PageCache\Model\Spi\PageCacheTagsPreprocessorInterface;
+use function array_merge;
+use function array_unique;
+use function implode;
 
 /**
  * Append cacheable pages response headers.
@@ -67,7 +72,7 @@ public function __construct(
      * @param Layout $subject
      * @return void
      */
-    public function afterGenerateElements(Layout $subject)
+    public function afterGenerateElements(Layout $subject): void
     {
         if ($subject->isCacheable() && !$this->maintenanceMode->isOn() && $this->config->isEnabled()) {
             $this->response->setPublicHeaders($this->config->getTtl());
@@ -78,22 +83,22 @@ public function afterGenerateElements(Layout $subject)
      * Retrieve all identities from blocks for further cache invalidation.
      *
      * @param Layout $subject
-     * @param mixed $result
-     * @return mixed
+     * @param string $result
+     * @return string
      */
-    public function afterGetOutput(Layout $subject, $result)
+    public function afterGetOutput(Layout $subject, string $result): string
     {
         if ($subject->isCacheable() && $this->config->isEnabled()) {
             $tags = [];
             $isVarnish = $this->config->getType() === Config::VARNISH;
 
+            /** @var BlockInterface[] $block */
             foreach ($subject->getAllBlocks() as $block) {
-                if ($block instanceof IdentityInterface) {
-                    $isEsiBlock = $block->getTtl() > 0;
-                    if ($isVarnish && $isEsiBlock) {
-                        continue;
+                if (!$isVarnish || ($block instanceof DataObject && !$block->getData('ttl'))) {
+                    $tags[] = (array) $block->getData('cache_tags');
+                    if ($block instanceof IdentityInterface) {
+                        $tags[] = $block->getIdentities();
                     }
-                    $tags[] = $block->getIdentities();
                 }
             }
             $tags = array_unique(array_merge([], ...$tags));