IPv6 support

[imobachgs]

Problem

Support for IPv6 is missing.

Solution

Initially, this PR implemented an additional IPv6 interface to support IPv6 settings. However, after some discussion, we decided to simplify the D-Bus interface and the schema as much as possible, keeping all IP configuration under a single org.opensuse.Agama1.Network.IP interface.

An example

  "network": {
    "connections": [
      {
        "id": "Ethernet network device 1",
        "method4": "manual",
        "method6": "manual",
        "interface": "eth0",
        "addresses": [
          "192.168.122.100/24",
          "::ffff:c0a8:7ac7/64"
        ],
        "gateway4": "192.168.122.1",
        "gateway6": "::ffff:c0a8:7a01",
        "nameservers": [
          "192.168.122.1",
          "2001:4860:4860::8888"
        ]
      }
    ]

Notes

This is an alternative to #758.

Testing

• Added a new unit test
• Tested manually

To do

• Add support for IPv6
• Unify IPv4 and IPv6 interfaces
• Adapt the schema
• Adapt the examples
• Fix the documentation
• Add more tests

[coveralls]

coverage: 72.381% (+0.1%) from 72.255% when pulling aac2a36 on imobachgs:ipv6-support into 4005226 on openSUSE:master.

[teclator]

LGTM, just added some suggestions but np ones.