[FR]: Add multi-app support for Firebase Analytics

### Description

Please add support in the Unity Firebase SDK for obtaining a FirebaseAnalytics instance tied to a specific FirebaseApp, similar to other modules like `FirebaseAuth.GetAuth(app)` and `FirebaseRemoteConfig.GetInstance(app)`.

### API Proposal

```cs
namespace Firebase.Analytics
{
public class FirebaseAnalytics
{
/// Returns the FirebaseAnalytics instance for the given FirebaseApp.
public static FirebaseAnalytics GetAnalytics(FirebaseApp app);

/// Logs an event to Firebase Analytics.
public void LogEvent(string name);

/// Static method to use the default app instance
public static void LogEvent(string name, Parameter[] parameters);
}
}
```

Usage:

```cs
FirebaseApp customApp = FirebaseApp.Create(new AppOptions() { /* ... */ }, "CustomApp");
FirebaseAnalytics analytics = FirebaseAnalytics.GetAnalytics(customApp);

analytics.LogEvent("custom_event");
```

### Firebase Product(s)

Analytics

### Targeted Platform(s)

All platforms

Start by comparing the proposed API with the existing FirebaseAuth.GetAuth(app) and FirebaseRemoteConfig.GetInstance(app) patterns. Then trace the Firebase Analytics entry points for the targeted platforms. Done means a custom FirebaseApp can obtain its own analytics instance while the proposed default-app static logging API continues to work on all platforms.

Written by the indexing model from the issue text.