feat: Add MLDv2 query response support

[bergzand]

This add support for responding to MLDv2 Listener Queries. Both general queries and multicast specific queries are supported. The implementation tries to match the igmpv2 implementation where the spec allows it.

• Querier's robustness variable is not handled
• Multicast address and source specific queries are treated as regular multicast address specific queries.
• Maximum Response Delays above 32768 are not calculated to spec.
• Delay between the query and the report is not random, but a simple fixed delay proportional to the Maximum Response Code

I've tested this via the multicast6 example with a Linux bridge device configured with multicast_querier enabled. Multicast traffic outside the subscriptions (such as neighbor solicitations for other solicited node multicast addresses) are filtered at the bridge, and subscribed addresses are forwarded to the tap device as expected.
Unfortunately this also means only the general listener queries are tested, I'm not sure how to test multicast address specific queries at this point.

Open questions

• Any automated tests that can and should be added to this pr?
• @thvdveld Is there anything in this code that needs some exception on ieee802.15.4 links?

[thvdveld]

Thank you for adding this!

Delay between the query and the report is not random, but a simple fixed delay proportional to the Maximum Response Code.

InterfaceInner holds a random number generator which you can use I think.

Any automated tests that can and should be added to this pr?

Maybe you could add something in src/iface/interface/tests/ipv6.rs? You could check if process_mldv2 reacts correctly to an incoming packet?

Is there anything in this code that needs some exception on ieee802.15.4 links?

I don't think so. It should behave similarly. Unless I missed an RFC.