Bug 25097: Add option to message_queue to allow for only specific sending notices

Bug 25097: Add option to message_queue to allow for only specific sending notices

The message_queue.pl currently will send all the messages queued up.  It would be nice to have some control over which messages are to be sent.

Example: We have libraries who only wanted to send 'hold notices' but not overdue, advance notice, etc during the covid-19 quarantine.  And some that were the opposite - turn off hold notices, but send other generated notices.

Test Plan:
1) Apply this patch
2) Enqueue some messages with different letter codes
3) Run process_message_queue.pl with the new -c parameter using one of the letter codes enqueued
4) Note that only the messages with the selected code were processed!

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>