Adding multipart responses into http4k

http4k.ks

// RFC: https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
interface Response {
  fun multipartBoundary(boundary: String): Response
  
  fun multiparts(): List<Response>
    
  //
  // The returned Response will use a content type of multipart response with 
  // a boundary string determined by the implementation.
  // 
  // If you want to use a custom boundary string, then you should first set 
  // the boundary in the (multipart response) content type of this response.
  //
  fun multiparts(responses: List<Response>): Response
}