code: public static String getRecurlySignature(String privateJsKey, List<String> extraParams) {
          final long unixTime = System.currentTimeMillis() / 1000L;
          final String uuid = UUID.randomUUID().toString().replaceAll("-", "");
          return getRecurlySignature(privateJsKey, unixTime, uuid, extraParams);
      }
docs: Get Recurly.js Signature
  See spec here: https://docs.recurly.com/deprecated-api-docs/recurlyjs/signatures
  <p>
  Returns a signature key for use with recurly.js BuildSubscriptionForm.
  
  @param privateJsKey recurly.js private key
  @param extraParams extra parameters to include in the signature
  @return signature string on success, null otherwise