Client support for file upload with Hilla endpoints

[platosha]

Extracted from #3130

When given at least one parameter of type File, the endpoint client should use multipart/form-data request type and include all parameters with their name, including all files.

Unit-testing is enough for the client part.

Example 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>;
}