feat(ads): define AdProvider abstract class

- Create AdProvider as an abstract class to define the interface for ad network providers
- Add initialize() method for ad network SDK setup
- Add loadNativeAd() method for loading native ads
- Include documentation comments for better code understanding

Author: fulleni

lib/ads/ad_provider.dart

@@ -0,0 +1,28 @@
+import 'package:google_mobile_ads/google_mobile_ads.dart';
+
+/// {@template ad_provider}
+/// An abstract class defining the interface for any ad network provider.
+///
+/// This abstraction allows the application to integrate with different
+/// ad networks (e.g., AdMob, Meta Audience Network) through a common API,
+/// promoting extensibility and decoupling.
+/// {@endtemplate}
+abstract class AdProvider {
+  /// {@macro ad_provider}
+  const AdProvider();
+
+  /// Initializes the ad network SDK.
+  ///
+  /// This method should be called once at application startup.
+  /// It handles any necessary setup for the specific ad network.
+  Future<void> initialize();
+
+  /// Loads a native ad.
+  ///
+  /// Returns a [NativeAd] object if an ad is successfully loaded,
+  /// otherwise returns `null`.
+  Future<NativeAd?> loadNativeAd();
+
+  // Future methods for other ad types (e.g., interstitial, banner)
+  // can be added here as needed in the future.
+}