Add IPv4Address.ipv6_mapped attribute

[wbolster]

Feature or enhancement

Add IPv4Address.ipv6_mapped attribute for symmetry with existing IPv6Address.ipv4_mapped

Pitch

The IPv6Address.ipv4_mapped attribute (docs) makes it easy to decode IPv6 addresses with embedded IPv4 addresses as defined in RFC4291.

It would be useful to also create its mirror version: IPv4Address.ipv6_mapped

If others also think this makes sense, please let me know and i can cook up a pull request.

[encukou]

This was added in #109466
Sorry that we missed this report!