Support MultipartFile parameters in Hilla generator

[platosha]

Extracted from #3130

For parameters of type MultipartFile, the expected client-side TypeScript type is File.

Let us not generate a form model for MultipartFile parameters. At this point, the form binding is out-of-scope.

Some example endpoints:

@BrowserCallable
class AppEndpoint {
  public void saveFile(MultipartFile file) {
  }

  public String saveFileAsUrl(MultipartFile file) {
    file.transferTo(...);
    return "https://...";
  }

  public void saveUserWithAvatar(UserData user, MultipartFile avatar) {
  }

  public void saveAvatarWithThumbnail(MultipartFile avatar, MultipartFile thumbnail) {
  }

  public ValueSignal<FileProcessingProgress> saveAndConvertHugeFile(MultipartFile hugeFile) {
  }
}