Streaming for multipart/form-data

[ymmt2005]

The current ogen generated code uses *Request.ParseMultipartForm, which stores data in the filesystem.
As we sometimes do not want to store data in the filesystem, it would be great if ogen
could generate code using *Request.MultipartReader which can process the request body as a stream.

[ymmt2005]

This would generate a fundamentally different interface from the non-streaming version.
I think we should add an extension x-ogen-multipart-streaming, just like we do for JSON streaming.

    post:
      requestBody:
        content:
          multipart/form-data:
            x-ogen-multipart-streaming: true

[k1ng440]

This would be great.