Fix: model json serializable generation (#1055)

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## Description
This is related to the following issue reported on the Flutter
repository flutter/flutter#122899

When running `build_runner` to generate the generated code of the models
from the api, it was causing issues with some of the api tests.

- Changed return type of `NewsBlockConverter()` from List<Map<String,
dynamic>> `List<dynamic>`. This was causing tests to fail.
- Added `explicitToJson` statements to some models that had complex
attributes inside, like another model. This was also causing tests to
fail.
- Since generated files are pushed to the repo, updated the .g files
with the new generated ones.


<!--- Describe your changes in detail -->



## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [ ] ✨ New feature (non-breaking change which adds functionality)
- [x] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [ ] 📝 Documentation
- [ ] 🗑️ Chore

flutter_news_example/api/lib/src/data/models/subscription.dart

@@ -7,7 +7,7 @@ part 'subscription.g.dart';
 /// A news subscription object which contains
 /// metadata about a subscription tier.
 /// {@endtemplate}
-@JsonSerializable()
+@JsonSerializable(explicitToJson: true)
 class Subscription extends Equatable {
  /// {@macro subscription}
  const Subscription({

flutter_news_example/api/lib/src/data/models/user.dart

@@ -7,7 +7,7 @@ part 'user.g.dart';
 /// {@template user}
 /// A user object which contains user metadata.
 /// {@endtemplate}
-@JsonSerializable()
+@JsonSerializable(explicitToJson: true)
 class User extends Equatable {
  /// {@macro user}
  const User({required this.id, required this.subscription});

flutter_news_example/api/lib/src/models/current_user_response/current_user_response.dart

@@ -7,7 +7,7 @@ part 'current_user_response.g.dart';
 /// {@template current_user_response}
 /// A response object which contains the current user.
 /// {@endtemplate}
-@JsonSerializable()
+@JsonSerializable(explicitToJson: true)
 class CurrentUserResponse extends Equatable {
  /// {@macro current_user_response}
  const CurrentUserResponse({required this.user});