ip: Support configuring per-device IPv4 sysctl forwarding

This feature introduces the ability to configure
`net.ipv4.conf.<interface>.forwarding` through nmstate.

Example YAML:

```yml
---
interfaces:
  - name: eth1
    type: ethernet
    state: up
    ipv4:
      address:
        - ip: 192.0.2.251
          prefix-length: 24
      dhcp: false
      enabled: true
      forwarding: true
```

Unit and integration test cases included.

Resolves: https://issues.redhat.com/browse/RHEL-36429

Changes by Gris Ge:
 * Use `version-info` D-BUS API for checking support status of IPv4
   forwarding in NetworkManager instead of comparing version number.
 * Add integration test case for enabled ipv4 forwarding on auto
   interface without DHCP server.

Signed-off-by: Wen Liang <liangwen12year@gmail.com>
Signed-off-by: Gris Ge <fge@redhat.com>