pvsecret: Support retrieving a secret by index

Allow users to retrieve a secret by its index. This allows users to
extract a specific secret in case of multiple ids.

Reviewed-by: Finn Callies <fcallies@linux.ibm.com>
Tested-by: Finn Callies <fcallies@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pvsecret/src/cli.rs

@@ -351,6 +351,8 @@ pub enum RetrInpFmt {
     Hex,
     /// Use a name-string. Will hash it if no secret with the name found.
     Name,
+    /// Use the secret-index (base 10) instead of the secret-ID
+    Idx,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, ValueEnum, Debug, Default)]