Add export library file

Since even in your example, you generate the kiota client in a separate package, it would be nice to generate a file that exports all the generated files into a library.

example:

```dart
library;

export 'generated/client.dart';
export 'generated/models/test.dart';
export 'generated/api/api_request_builder.dart';
export 'generated/api/v1/test/test_request_builder.dart';
...
```

Start by tracing the Dart client generation flow and how generated/client.dart, generated/models/test.dart, generated/api/api_request_builder.dart, and generated/api/v1/test/test_request_builder.dart are selected. Done means the generator emits a library file that exports all generated files, following the example in the issue.

Written by the indexing model from the issue text.