code: public static SignatureUtil buildInstance(String privateKey, String publicKey,
                                                Algorithms algorithms) {
          return new SignatureUtilImpl(privateKey, publicKey, algorithms);
      }
docs: 构建一个SignatureUtil
  
  @param privateKey PKCS8文件格式的私钥
  @param publicKey  X509格式的公钥
  @param algorithms RSA加密类型
  @return SignatureUtil