interfaces: settings: convert to MVC

[swhite2]

• Includes a migration.
• The LLT and LL DUID generation used to calculate based on the MAC of the remote client currently connected to the webgui. I believe this is wrong (and in a lot of cases it won't be in the ARP/NDP table), as the DUID is supposed to be the system-wide unique identifier, so it now picks the system primary interface:

$primary_if = get_primary_interface_from_list();
$system_mac = get_interface_mac(get_real_interface($primary_if));

• I haven't touched the config.xml.sample, as it doesn't have the OPNsense namespace yet, is it enough to just add it there?
• Includes the dhcp6_ratimeout as @fichtner mentioned in interfaces: convert settings page to MVC/API #9141 (comment)

Closes #9141