networkd/dhcp6: Add support for IPv6 SIP servers (#38269)

This pull-request adds support for IPv6 SIP servers, which can be
obtained from DHCPv6 options.

[RFC3319](https://www.rfc-editor.org/rfc/rfc3319.html) defines two
options for this: option 21 (SIP Servers Domain Name List) and option 22
(SIP Servers IPv6 Address List).

systemd-networkd currently only supports IPv4 SIP servers, but ISPs like
NTT EAST provide IPv6 SIP servers. Exposing this in the D-Bus API would
allow other programs, such as downstream DHCPv6 servers, to utilize it.

Usage:

```
[DHCPv6]
UseSIP=yes
```

Partial output of `run0 systemd-networkd-tests.py
NetworkdDHCPClientTests.test_dhcp_client_use_sip`:

```console
> networkctl status veth99
● 17: veth99
                 NetDev File: /run/systemd/network/25-veth.netdev
                   Link File: /usr/lib/systemd/network/99-default.link
                Network File: /run/systemd/network/25-dhcp-client.network
                              └─/run/systemd/network/25-dhcp-client.network.d/override.conf
                       State: routable (configured)
                Online state: online
                        Type: ether
                        Kind: veth
                      Driver: veth
            Hardware Address: 12:34:56:78:9a:bc
                         MTU: 1500 (min: 68, max: 65535)
                       QDisc: noqueue
IPv6 Address Generation Mode: eui64
    Number of Queues (Tx/Rx): 2/2
            Auto negotiation: no
                       Speed: 10Gbps
                      Duplex: full
                        Port: tp
                     Address: 192.168.5.181 (DHCPv4 via 192.168.5.1)
                              2600::15
                              fe80::1034:56ff:fe78:9abc
                     Gateway: 192.168.5.1
                              fe80::1034:56ff:fe78:9abd
                         SIP: 192.168.5.1
                              2600::1
                              foo.example.com
           Activation Policy: up
         Required For Online: yes
            DHCPv4 Client ID: IAID:0x7ec6b6c/DUID
          DHCPv6 Client IAID: 0x7ec6b6c
          DHCPv6 Client DUID: DUID-EN/Vendor:0000ab11617740de1342c3a2
```

diff --cc src/libsystemd-network/sd-dhcp6-lease.c
Simple merge