isc-dhcp: move to plugins and change DHCPv6 default to Dnsmasq (no more automatic PD support)

[fichtner]

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Is your feature request related to a problem? Please describe.

Since ISC DHCP is EOL it needs to be moved to plugins. Kea and Dnsmasq both fill the void left by it.

Describe the solution you like

Move ISC DHCP to plugins and change the automatic IPv6 setup to Dnsmasq (factory reset and wizard default).

• adapted dhcpdv6 and radvd to only use track6 mode f8da6e1 8d0da12
• idassoc6 mode for track6 replacement without magic e790033
• default config.xml adaptation for Dnsmasq interfaces: make Dnsmasq the default IPv6 provider #9595
• Add pluggable glue for ISC dhcp (plugin install script) 4469cf2
• Add empty plugin for 25.7.11 opnsense/plugins@23997fecc opnsense/tools@84d6584f9
• Move files to plugin and attempt a core code cleanup where possible
• ISC DHCP IPv6 static mappings export is missing? isc-dhcp: add static mapping export in DHCPv6 page #9639
• Adjust console scripts setport.sh and setaddr.sh: setaddr cleared dhcp_ranges on no, but likely also clears IPv6 config introduced lately 261ad14 635dd1b b30630d
• Adjust wizard for this, better later in wizard: IPv6 configuration in the wizard #9653

Describe alternatives you considered

Use Kea, but it does not offer dynamic prefix tracking.

Additional context

N/A