Add type hints for new interface and impementation

Author: renttek

setup/src/Magento/Setup/Model/CryptKeyGenerator.php

@@ -34,7 +34,7 @@ public function __construct(Random $random)
      *
      * @throws \Magento\Framework\Exception\LocalizedException
      */
-    public function generate()
+    public function generate() : string
     {
         return md5($this->getRandomString());
     }

setup/src/Magento/Setup/Model/CryptKeyGeneratorInterface.php

@@ -19,5 +19,5 @@ interface CryptKeyGeneratorInterface
      *
      * @return string
      */
-    public function generate();
+    public function generate() : string;
 }