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

Start at the admin.initializeApp options described in the issue and trace how the SDK constructs URLs for OAuth2, App Check, FCM, and IID requests. Inventory the existing fixed endpoints and define how optional per-service URLs should be represented; the work is done when each listed service can route through its configured custom endpoint without requiring network-level proxy interception.

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.