[DX][Cache] Set right type hint for cacheItem on AdapterInterface getters

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Fixes missing type hint for cache item on pool getters so methods on Symfony CacheItem
is correclty suggested when using IDE's or api documentation.

As proposed here: symfony#19728 (comment)

Note: Specifically sets array of items as return type of getItems as phpdoc and IDEs supports
this by now, and since this is specifically what is being returned.

Author: André R

src/Symfony/Component/Cache/Adapter/AdapterInterface.php

@@ -21,4 +21,17 @@
  */
 interface AdapterInterface extends CacheItemPoolInterface
 {
+    /**
+    * {@inheritdoc}
+    *
+    * @return \Symfony\Component\Cache\CacheItem
+    */
+    public function getItem($key);
+
+   /**
+    * {@inheritdoc}
+    *
+    * return \Symfony\Component\Cache\CacheItem[]
+    */
+    public function getItems(array $keys = []);
 }