[REQ] Form parameters with additionalProperties

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

I have a POST route in my openapi specification with a form-urlencoded body. The schema of this body is composed of some known properties that are defined, but also should accept other arbitrary properties as a Map that will be passed to the service without prior validation. To do this, I have an object type with the additionalProperties property to true.

Currently, it generates this error :
Form parameters with additionalProperties are not supported by OpenAPI Generator. Please report the issue to https://github.com/openapitools/openapi-generator if you need help.

Describe the solution you'd like

I would like the generator to generate a method signature with the known parameters as classic parameters, with an additional parameter with a Map<String, Object> or Map<String, String> type

public ResponseEntity<ResponseDTO> postRoute(Boolean param1, String param2,
            @RequestParam Map<String, Object> additionalParameters) {
    ...
}

Describe alternatives you've considered

I have not thought of a better way to do this. Currently, I need to remove the @Override modifier because I need this Map.

I can't change the content type or define all the parameters, because I don't control the client that will call this route (a webhook).

Additional context

Here is my openapi specification for the requestBody I need :

  requestBody:
    required: true
    content:
      application/x-www-form-urlencoded:
        schema:
          type: object
          properties:
            param1:
              type: boolean
              default: false
            param2:
               type: string
          additionalProperties: true

Start by tracing how Java server generation handles application/x-www-form-urlencoded request bodies and where the “Form parameters with additionalProperties” error is emitted. Compare the generated method signature with the requestBody schema in the issue. Done means known properties remain typed parameters and arbitrary form fields are accepted through a Map parameter without the current error.