Add Value and ValueInterface to Magento Framework's public API

These classes are necessary/useful for creating a backend source for a
Magento Configuration field.  As such they should be reliable and part
of the public API.

Author: navarr

lib/internal/Magento/Framework/App/Config/Value.php

@@ -17,6 +17,8 @@
  * @method string getValue()
  * @method \Magento\Framework\App\Config\ValueInterface setValue(string $value)
  *
+ * @api
+ *
  * @SuppressWarnings(PHPMD.NumberOfChildren)
  */
 class Value extends \Magento\Framework\Model\AbstractModel implements \Magento\Framework\App\Config\ValueInterface

lib/internal/Magento/Framework/App/Config/ValueInterface.php

@@ -9,7 +9,7 @@
 
 /**
  * Interface \Magento\Framework\App\Config\ValueInterface
- *
+ * @api
  */
 interface ValueInterface
 {