network: Add support to advertie ipv6 route

network: Add support to advertie ipv6 route

Implements https://tools.ietf.org/html/rfc4191

cat veth99.network
```
[Match]
Name=veth99

[Network]
DHCP=no
IPv6PrefixDelegation=yes
Address=2001:db8:0:1::1/64

[IPv6Prefix]
Prefix=2001:db8:0:1::4/64

[IPv6RoutePrefix]
Route=2001:db0:fff::/48

```
Wireshark

```
Frame 481: 142 bytes on wire (1136 bits), 142 bytes captured (1136 bits) on interface 0
Ethernet II, Src: 1e:04:f8:b8:2f:d4 (1e:04:f8:b8:2f:d4), Dst: IPv6mcast_01 (33:33:00:00:00:01)
Internet Protocol Version 6, Src: fe80::1c04:f8ff:feb8:2fd4, Dst: ff02::1
Internet Control Message Protocol v6
    Type: Router Advertisement (134)
    Code: 0
    Checksum: 0xec77 [correct]
    [Checksum Status: Good]
    Cur hop limit: 0
    Flags: 0x00, Prf (Default Router Preference): Medium
    Router lifetime (s): 0
    Reachable time (ms): 0
    Retrans timer (ms): 0
    ICMPv6 Option (Source link-layer address : 1e:04:f8:b8:2f:d4)
        Type: Source link-layer address (1)
        Length: 1 (8 bytes)
        Link-layer address: 1e:04:f8:b8:2f:d4 (1e:04:f8:b8:2f:d4)
    ICMPv6 Option (MTU : 1500)
        Type: MTU (5)
        Length: 1 (8 bytes)
        Reserved
        MTU: 1500
    ICMPv6 Option (Prefix information : 2001:db8:0:1::4/64)
        Type: Prefix information (3)
        Length: 4 (32 bytes)
        Prefix Length: 64
        Flag: 0xc0, On-link flag(L), Autonomous address-configuration flag(A)
        Valid Lifetime: 2592000
        Preferred Lifetime: 604800
        Reserved
        Prefix: 2001:db8:0:1::4
    ICMPv6 Option (Route Information : Medium 2001:db0:fff::/48)
        Type: Route Information (24)
        Length: 3 (24 bytes)
        Prefix Length: 48
        Flag: 0x00, Route Preference: Medium
            ...0 0... = Route Preference: Medium (0)
            000. .000 = Reserved: 0
        Route Lifetime: 604800
        Prefix: 2001:db0:fff::
```