[csharp-netcore] Form data serialize non-primitive objects

Created by: Linux13524

In the current csharp-netcore generator objects passed as multipart form-data are treated as primitive type and converted to string. With this changes it will also be possible to pass non-primitive objects that will be serialized as JSON, similar to how it is done in the csharp generator: https://github.com/OpenAPITools/openapi-generator/blob/eeb1711f9f073a90b62e4d0fa2ecf81b51a13281/modules/openapi-generator/src/main/resources/csharp/api.mustache#L272

Example specification supported now with the changes:

...
paths:
  /upload:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              $ref: '#/components/schemas/UploadsRequest'

components:
  schemas:
    UploadsRequest:
      type: object
      properties:
        config:
          $ref: '#/components/schemas/UploadConfig'
        file:
          type: string
          format: binary

    UploadConfig:
      type: object
      properties:
        version:
          type: string
        timeout:
          type: integer
          format: int32

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• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
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