IPv6 support (multicast and MAC address filtering)

[oddstr13]

Depends on upstream issue georgerobotics/cyw43-driver#18

See my downstream work so far in earlephilhower/arduino-pico#732

[peterharperuk]

I'm not sure if this is required once this is merged? georgerobotics/cyw43-driver#24

[peterharperuk]

Marking as complete - reopen if you disagree.