[dart][dart-dio][dart-dio-next] Add support for classes that extend EnumClass to encodeCollectionQueryParameter

Created by: tfbsza

Is your feature request related to a problem? Please describe.

Recent additions to the dart-dio and dart-dio-next generators (https://github.com/OpenAPITools/openapi-generator/commit/fdbf02764d144ff1469ce3905dff5b19352b6447) fixed encoding query parameters correctly. It doesn't seem to support classes that extend EnumClass though.

Describe the solution you'd like

Please add support for classes that extend EnumClass.

Describe alternatives you've considered

As a temporary fix, I manually changed line 64 of generated file api_util.dart which fixed the issue in the meantime:

Changed from: ListParam<T> encodeCollectionQueryParameter<T>

Changed To: ListParam<String> encodeCollectionQueryParameter<T>