async function generateRefreshToken() {
  const refreshToken = (await randomBytes(Math.ceil(REFRESH_TOKEN_LENGTH / 2)))
    .toString('hex')
    .slice(0, REFRESH_TOKEN_LENGTH);
  const refreshTokenHash = hashRefreshToken(refreshToken);

  return {
    refreshToken,
    refreshTokenHash,
  };
}
@private
@description Generate a refresh token and its hash.
@example
const { refreshToken, refreshTokenhash } = await generateRefreshToken();
@returns {Promise} Resolving with refreshToken and refreshTokenHash.