network: sd-radv - Introduce pref64 support (RFC8781)

network: sd-radv - Introduce pref64 support (RFC8781)

Implements: https://datatracker.ietf.org/doc/html/rfc8781

```

[IPv6PREF64Prefix]
Prefix=2003:da8:1:0::/64
ValidLifetimeSec=30m

Frame 16: 126 bytes on wire (1008 bits), 126 bytes captured (1008 bits) on interface veth99, id 0
Ethernet II, Src: 06:c7:41:95:1d:7f (06:c7:41:95:1d:7f), Dst: IPv6mcast_01 (33:33:00:00:00:01)
Internet Protocol Version 6, Src: fe80::4c7:41ff:fe95:1d7f, Dst: ff02::1
Internet Control Message Protocol v6
    Type: Router Advertisement (134)
    Code: 0
    Checksum: 0x0ca0 [correct]
    [Checksum Status: Good]
    Cur hop limit: 0
    Flags: 0x00, Prf (Default Router Preference): Medium
    Router lifetime (s): 1800
    Reachable time (ms): 0
    Retrans timer (ms): 0
    ICMPv6 Option (Source link-layer address : 06:c7:41:95:1d:7f)
    ICMPv6 Option (Prefix information : 2002:da8:1::/64)
    ICMPv6 Option (PREF64 Option)
        Type: PREF64 Option (38)
        Length: 2 (16 bytes)
        0000 0111 0000 1... = Scaled Lifetime: 225
        .... .... .... .001 = PLC (Prefix Length Code): 64 bits prefix length (0x1)
        Prefix: 64:ff9b::

```