MDL-36580 backup: General support for encrypted contents in backups

- Built using standard backup custom fields.
- Can be applied potentially everywhere.
- Automatically addded 'encrypted' attribute.
- Defaults to site generated key.
- Enforces key robutness / provides authentication (hmac integrity)
- Covered with unit tests.

backup/backup.class.php

@@ -141,6 +141,15 @@ abstract class backup implements checksumable {
      * Usually same than major release zero version, mainly for informative/historic purposes.
      */
     const RELEASE = '3.4';
+
+    /**
+     * Cipher to be used in backup and restore operations.
+     */
+    const CIPHER = 'aes-256-cbc';
+    /**
+     * Bytes enforced for key, using the cypher above. Restrictive? Yes, but better than unsafe lengths
+     */
+    const CIPHERKEYLEN = 32;
 }
 
 /*