Add IFormData mixin to send form data via vexana.

[MehmetKaranlik]

@VB10

• form data sending ability is provided.

class UploadSoundModel extends INetworkModel with IFormDataModel {
  final FormData? files;

  UploadSoundModel({
    this.files,
  });

  @override
  UploadSoundModel fromJson(Map<String, dynamic> json) {
    return UploadSoundModel(
      files: json['files'] as FormData?,
    );
  }

  @override
  Map<String, dynamic>? toJson() {
    return {
      'files': files,
    };
  }

  @override
  Future<bool> uploadSound(UploadSoundModel model) async {
    final response = await manager.send<EmptyModel, EmptyModel>(
      'api/posts/multi-upload',
      parseModel: EmptyModel(),
      method: RequestType.POST,
      data: model,
    );

    return response.data != null;
  }