Support Dart core types in the return types.

[dkaera]

I've noticed that it is a pretty common issue when you need to fetch only one column value like in this issue, but in the current version of Floor you have to write a separate database-view.
My suggestion is to support Dart core types in the DAO's just by defining in the return types. Thus, the generated code will look like this:

  @override
  Future<List<int?>> findAllTaskId() async {
    return _queryAdapter.queryList(
      'SELECT id FROM task',
      mapper: (Map<String, Object?> row) {
        return row['id'] as int?;
      },
    );
  }

[dkaera]

Just pushed a first realization here
If in doubt, welcome to the discussion.

[dkaera]

Hey @ThomasMiddel I think MR #691 is ready for review :)

[dkaera]

@ThomasMiddel This feature has been done.
In the meantime, I realized that in the context of this feature we need to support TypeConverter types and Enums as return types. Added with the required tests. Welcome for review :)