Gracefully handle missing GoogleService-Info.plist on iOS

**Is your feature request related to a problem? Please describe.**

On Android, if you don't call `await PushNotifications.register();` on JS side, the app would still work fine without push notifications.

However, on iOS, `FirebaseApp.configure()` always gets called. That crashes the app when `GoogleService-Info.plist` is missing:
https://github.com/capacitor-community/fcm/blob/fab6ec2310a6721513f0efc67cc75e8da1636729/ios/Plugin/Plugin.swift#L19-L22

This is not ideal for making the push notification feature optional for some environments, such as for some of the devs on the team.

**Describe the solution you'd like**

Offer a configuration or an API change to turn off this behavior.

One option could be using the `didRegisterForRemoteNotificationsWithDeviceToken` hook Capacitor PushNotifications plugin uses, and let the user add the `FirebaseApp.configure()` there, if possible.

One would be to simply remove `FirebaseApp.configure()` as the official Capacitor guide makes the user add that call. So, the user would have control over the call and optionally not call it.
https://capacitorjs.com/docs/guides/push-notifications-firebase#add-initialization-code
> ... and then add the configuration method for Firebase to initialization code to your AppDelegate.swift file, in the application(didFinishLaunchingWithOptions) method.
> FirebaseApp.configure()

**Describe alternatives you've considered**
On Android, you can get away by not calling `PushNotifications.register()`. But, I couldn't find a workaround or alternative for iOS.

**Additional context**


Start with ios/Plugin/Plugin.swift at the FirebaseApp.configure() call and compare it with the Capacitor PushNotifications registration hook and the AppDelegate application(didFinishLaunchingWithOptions) guidance linked in the issue. Determine a supported way to avoid configuration when GoogleService-Info.plist is absent, then verify that iOS apps without Firebase configuration no longer crash while configured apps still initialize Firebase.

Written by the indexing model from the issue text.