tape: implement encrypted backup - simple version

This is just a proof of concept, only storing the encryption key fingerprint
inside the media-set label.

src/api2/types/tape/media.rs

@@ -74,6 +74,9 @@ pub struct MediaIdFlat {
     /// MediaSet Creation time stamp
     #[serde(skip_serializing_if="Option::is_none")]
     pub media_set_ctime: Option<i64>,
+    /// Encryption key fingerprint
+    #[serde(skip_serializing_if="Option::is_none")]
+    pub encryption_key_fingerprint: Option<String>,
 }
 
 #[api()]