[FR]: Allow setting custom URLs (proxy) for Firebase Admin SDK services (OAuth2, AppCheck, FCM, IID, etc.)

从 issue 中描述的 admin.initializeApp 选项入手，跟踪 SDK 如何为 OAuth2、App Check、FCM 和 IID 请求构建 URL。盘点现有的固定 endpoint，并定义应如何表示可选的按 service 配置的 URL；当列出的每个 service 都可以通过其配置的自定义 endpoint 进行路由，而不需要网络级代理拦截时，工作即完成。

Is your feature request related to a problem? Please describe.
We are using Firebase Admin SDK in a private network environment where direct access to Firebase services is restricted.
Currently, the SDK uses fixed URLs for services like OAuth2, App Check, FCM, and IID.
Because of this, we cannot route traffic through a custom proxy or internal endpoint.

For example, in our private network setup, we need to use a proxy URL to reach these services, but the SDK doesn’t provide a way to override or customize these endpoints.

Describe the solution you'd like
We would like to have the ability to optionally set custom URLs for the different Firebase services used by the Admin SDK.

For instance, being able to configure a proxy URL or custom domain for:
• OAuth2 (+jwks)
• App Check
• FCM
• IID

This configuration could be done through the SDK initialization options, like:

admin.initializeApp({
  apiURLs: {
    oauth2: "https://proxy.example.com/oauth2",
    appCheck: "https://proxy.example.com/appcheck",
    fcm: "https://proxy.example.com/fcm",
    iid: "https://proxy.example.com/iid"
  }
});

This way, we can route all requests through our internal proxy without modifying the SDK code.

Describe alternatives you've considered
Currently, the only alternative is to set up a network-level proxy that intercepts outgoing requests.
However, this is not ideal because:
• It requires additional infrastructure and maintenance.
• It is harder to debug and trace traffic for each service.
• It does not allow fine-grained control over different service endpoints.

Additional context
This feature would greatly improve the flexibility of the SDK in environments with strict network policies.
It would also make the Node.js SDK more consistent with the Go SDK’s approach, while extending the idea to cover all relevant Firebase services.