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)

首先检查现有的 BYOIP 行为以及关联的 issue #11527；该 issue 描述了向公共网络添加多个 IPv6 地址。实现前，明确 API、分配规则，以及受影响的公共网络或访客网络流程。完成的标准是：在两种受支持的网络上下文中，都可以将 IPv6 地址分配给特定账户或域，并覆盖由此产生的行为。

由索引模型根据 Issue 内容生成。