File upload support for Hilla endpoint controller

[platosha]

Add runtime support for parameters of type MultipartFile in Hilla endpoint controller.

The files should be uploaded using HTTP multipart/form-data encoding. Note that the encoding changes from the regular application/json when requests contain file parameters.

Users can add additional parameters and return any Hilla supported result type in the same endpoint.

At the same time, let us add integration tests.

Example endpoint:

@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) {
  }
}

TypeScript usage:

export default function UploadForm({}) {
  const [uploadState, setUploadState] = useState("");

  const uploadFile = useCallback(async (event: SubmitEvent) => {
    event.preventDefault();
    const file = new FormData(event.target).get("file");
    await AppEndpoint.saveFile(file);
    setUploadState("ok!");
  });

  return <form onSubmit={uploadFile}>
    <input type="file" name="file" />
    <button type="submit">Upload</button>
    <output>{uploadState}</output>
  </form>;
}