Description of problem:

Nmstate user might not know IPv6 has `autconf` property, so they might only have

```
ipv6:
enabled: true
dhcp: true
```

The state nmstate applied is not what customer desired as DHCPv6 does not have IPv6 routing information.

Hence nmstate should generate a warning message to user indicating nmstate is auto-fixing the desire state by enabling autoconf also.

Version-Release number of selected component (if applicable):
nmstate-2.2

How reproducible:
100%

Steps to Reproduce:
1. Use nmstate to apply this YAML:

```yml
—
interfaces:

• name: eth1
  state: up
  ipv6:
  enabled: true
  dhcp: true
  ```
  2.
  3.

Actual results:

Only DHCPv6 enabled on this interface. Hence no IPv6 routing.

Expected results:

Both autoconf and DHCPv6 are enabled.

Additional info:

Acceptance Criteria:
Given a System Administrator is utilizing nmstate-2.2 to configure network settings,

When the YAML configuration (see description) to enable IPv6 with DHCPv6 is applied,

Then nmstate should generate a warning message to the System Administrator indicating the necessity of enabling autoconf along with DHCPv6 for proper IPv6 routing. It should then automatically enable autoconf if it's not specified in the configuration.

Definition of done

• The implementation meets the acceptance criteria
• Unit test and integration test are written and passed
• The code is part of a build attached to an errata