Allow root admin to configure DNS servers on behalf of a specific user/domain and support project-scoped DNS servers

先檢視現有的 DNS 伺服器設定流程和 PR #13821，接著追蹤 account、domain、project 以及 root-admin 的擁有權如何表示。當 root admin 可以為使用者、domain 或 project 配置 DNS 伺服器，而已配置的使用者和 domain 無法變更其 URL 時，即表示完成；範圍和權限規則應由適當的測試涵蓋。

The required feature described as a wish

Currently, a DNS server can only be set up for the calling account/domain, there is no way for a root admin to configure one on behalf of a different account or domain, and no way for a DNS server to belong to a project.

As a root admin, I would like to be able to:

1. Set up a DNS server on behalf of a specific user/account
2. Set up a DNS server on behalf of a specific domain
3. Attach a DNS server to a project

Additional constraint:

4. If a DNS server was set up by root admin on behalf of a user/domain, that user/domain should not be able to modify its URL themselves. Only root admin (or whoever provisioned it) should be able to change it.

Motivation:

Asks 1 and 2 are primarily driven by the recent restriction that only root admin can configure a DNS server on a private/internal (RFC1918) address, domain admins and regular users can no longer do so directly (see PR #13821). Without a way for root admin to provision such a server on behalf of another account/domain, that use case is lost entirely for non-root-admin users.

Ask 3 (project support) wasn't part of the initial scope for this feature. It's being tracked here as a follow-up enhancement rather than a gap in the original design.

Ask 4 follows directly from 1 and 2: if the owning user/domain could freely change the URL afterward, they could redirect an admin-provisioned DNS server anywhere they like, defeating the purpose of restricting private-address setup to root admin in the first place.