Add function to list all files in a bucket

Add functionality to list all the files in a bucket. Should behaviour like so:

```dart
import 'package:nitric_sdk/nitric.dart';

final assets = Nitric.bucket("assets").requires([
  BucketPermission.reading,
  BucketPermission.deleting,
]);

final files = await assets.files();

await Future.wait(files.map((file) async {
  await file.delete();
}))
```