code: public static HMac hmac(HmacAlgorithm algorithm, String key) {
  		return new HMac(algorithm, StrUtil.utf8Bytes(key));
  	}
docs: 创建HMac对象，调用digest方法可获得hmac值
  
  @param algorithm {@link HmacAlgorithm}
  @param key 密钥，如果为<code>null</code>生成随机密钥
  @return {@link HMac}
  @since 3.3.0