IPv6 Assign an Account or DOMAIN as IPv4 - BYOIPv6 feature as part of the BYOIP

### The required feature described as a wish

As a User/Admin/Operator I would like to Assign IPv6 to a Specific Domain for the Public or Guest networks, This to be aligned with the BYOIP , We can now add multiple IPv6 to the public (https://github.com/apache/cloudstack/issues/11527)

Start by reviewing the existing BYOIP behavior and the linked issue #11527, which describes adding multiple IPv6 addresses to the public network. Clarify the API, allocation rules, and affected public or guest network flows before implementation. Done means an IPv6 address can be assigned to a specific account or domain in both supported network contexts, with coverage for the resulting behavior.

Written by the indexing model from the issue text.