UI: Assign Instance Group to Domain/Account during Creation

### The required feature described as a wish

As an Admin or Operator, I would like to be able to Instance Group to Domain/Account in the UI. Currently the only way to do this is by logging into a user in account that you want the instance group to be created in.

首先定位用于创建 Instance Group 的 UI 入口，并检查当前如何选择 Domain 或 Account 上下文。在定义变更之前，确认现有的创建流程和权限。完成的标准是，Admin 或 Operator 可以在创建期间将 Instance Group 分配给 Domain 或 Account，而无需登录该 account。

由索引模型根据 Issue 内容生成。