Wake on LAN: Support IPv6-only interfaces

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• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I have searched the existing issues, open and closed, and I'm convinced that mine is new.
• When the request is meant for an existing plugin, I've added its name to the title.

Is your feature request related to a problem? Please describe.
I have an interface on my OPNsense firewall that only has an IPv6 address and no IPv4 address, and I would like to broadcast Wake on LAN magic packets on it.

Describe the solution you'd like
I would like the Wake on LAN plugin to consider IPv6 addresses on interfaces and send broadcast packets to an iPv6 multicast address (e.g. ff02::1) if an IPv6 address is available.

Describe alternatives you've considered
Do not send an IP header at all, but rather send a raw ethernet frame with the magic packet as the payload.

Additional context
None

Start by locating the Wake on LAN plugin and reading how it discovers interface addresses and sends magic packets. Check how an IPv6-only interface should use the proposed multicast destination, then verify that the plugin sends the packet successfully without IPv4; the payload names no specific files or tests.