[dart][dart-dio-next] File arrays in Form parameter are generated as a single file in the api

Created by: ahmednfwela

Description

form parameters with file arrays generated according to OAS3 are generated as a single MultipartFile instead of List<MultipartFile> (using BuiltList<MultipartFile> here is useless, since it doesn't get serialized)

openapi-generator version

5.2.0

OpenAPI declaration

      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                files:
                  type: array
                  items:
                    type: string
                    format: binary