T5518: Add basic MLD support

Currently VyOS has `protocol igmp` option to enable IGMP querier and reports through FRR's pimd.

I would like to add support for IPv6 as well since FRR's IPv6 multicast functionality has significantly improved.

Enabling both MLD and IGMP on a VyOS router will allow us to turn on multicast snooping on layer-3 switches in dual-stack networks.

Example commands:

```
// Enable on interface eth0
set protocols pim6 interface eth0

// Explicitly join multicast group ff18::1234 on interface eth1
set protocols pim6 interface eth1 mld join ff18::1234

// Explicitly join source-specific multicast group ff38::5678 with source address 2001:db8::1 on interface eth1
set protocols pim6 interface eth1 mld join ff38::5678 source 2001:db8::1
```

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