iptunnel: introduce support for ipip/sit/ip6ip6/ipip6 tunnels

Example usage:
```yml
---
interfaces:
  - name: ipip0
    type: ip-tunnel
    state: up
    ip-tunnel:
      type: ipip
      local: 192.0.2.1
      remote: 192.0.2.2
      ttl: 255
```

Unit and integration test cases included.

Closes #2835
Resolves: https://issues.redhat.com/browse/RHEL-79801

Signed-off-by: Jan Vaclav <jvaclav@redhat.com>