feat: add setFirebaseOptions() for runtime multi-tenant Firebase configuration

[Aravinthan-J]

Summary

Adds a setFirebaseOptions() method that allows apps to initialize Firebase with credentials provided at runtime, rather than relying solely on static google-services.json / GoogleService-Info.plist files baked into the binary.

This unblocks multi-tenant SaaS apps where each tenant operates their own Firebase project and the correct credentials are only known after the user authenticates.

Closes #60

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Motivation

In multi-tenant apps, push notification tokens must be scoped to the correct Firebase project per tenant. There is currently no way to do this with this plugin — the only path was to either:

• Ship a separate binary per tenant (impractical for SaaS)
• Maintain a fork of this plugin

This PR upstreams the implementation from an internal fork that has been running in production on Capacitor 5/6/7.

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Changes

src/definitions.ts

• Adds FirebaseOptions interface
• Adds setFirebaseOptions(options: FirebaseOptions): Promise<void> to FCMPlugin

src/web.ts

• Adds setFirebaseOptions() stub (throws unimplemented — native-only)

Android (FCMPlugin.java)

Creates a named secondary FirebaseApp ("FCM") so it does not conflict with or re-initialize any default app already present from google-services.json. All getToken() / refreshToken() / setAutoInit() / isAutoInitEnabled() calls are automatically routed through the secondary app when set.

Note: gcmSenderId is accepted for interface parity with iOS but omitted from FirebaseOptions.Builder — setGcmSenderId() was removed in Firebase Android SDK 29+.

iOS (FCMPlugin.swift)

• load() now checks for the presence of GoogleService-Info.plist before calling FirebaseApp.configure(), so apps without a plist don't crash on startup
• setFirebaseOptions() configures the default Firebase app with runtime credentials; fails fast with a clear error if Firebase is already initialized (Firebase SDK limitation — re-initialization is not supported on iOS)

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Usage

import { FCM } from '@capacitor-community/fcm';
import { PushNotifications } from '@capacitor/push-notifications';

// After login — fetch tenant config from your server
const config = await fetchTenantFirebaseConfig();

await FCM.setFirebaseOptions({
  applicationId: config.mobilesdk_app_id, // Android: mobilesdk_app_id / iOS: GOOGLE_APP_ID
  gcmSenderId:   config.project_number,
  apiKey:        config.apiKey,
  projectId:     config.project_id,
});

await PushNotifications.register();
const { token } = await FCM.getToken();
// token is now scoped to the tenant's Firebase project

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Platform behaviour summary

	Android	iOS
Mechanism	Named secondary FirebaseApp("FCM")	Configures default app at runtime
Coexists with static config file	Yes — secondary app is independent	No — do not ship GoogleService-Info.plist
Re-initialization (tenant switch)	Yes — reuses existing secondary app	No — app restart required (Firebase SDK limitation)

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Known limitations / follow-up

• iOS re-initialization: Once FirebaseApp.configure() is called it cannot be undone. Apps that need to switch tenants at runtime on iOS must restart.
• iOS secondary app for Messaging: The Firebase iOS SDK does not expose a public Messaging(app:) API, so getToken() on iOS always uses the default app (which is configured by setFirebaseOptions()).

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Testing

Tested on:

• Android (Capacitor 7, Firebase Android SDK 33)
• iOS (Capacitor 7, Firebase iOS SDK 11)

Existing behaviour is fully preserved when setFirebaseOptions() is never called.