Allow disabling ddns in reservations

Allow the use of ddns-send-updates (value false) at reservation scope.

I place my pool leases in dhcp.example.com. This subdomain is enabled for dynamic update. I place my fixed address leases (reservations) in example.com which is not enabled for dynamic update by directly editing my BIND 9 zone files. kea-dhcp4 should not send dynamic updates for my reservations. My pool is in 10.96.6.0/24. I use other /24's in the 10.96/16 netblock for different functions like printers and cameras. They all share the same netmask and default router so I can't put them in different subnets.

Currently, ddns is set on a per-subnet basis. It seems that allowing it to be disabled per reservation would achieve the desired result. Other solutions are welcome.

I'm a coder but not familiar with the Kea code base so I can help shake out an implementation but I'm not able to create one from scratch.

Replies here are fine. I'm also on kea-users and posted about it there.