api2/tape: add notify_user to backup(-jobs) and restore api calls

so that a user can be given that will be notified for
manual intervention (e.g. inserting a tape)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/server/email_notifications.rs

@@ -397,7 +397,7 @@ pub fn send_updates_available(
 }
 
 /// Lookup users email address
-fn lookup_user_email(userid: &Userid) -> Option<String> {
+pub fn lookup_user_email(userid: &Userid) -> Option<String> {
 
     use crate::config::user::{self, User};