1. Add spot cross margin API support
2. Add spot order operations using cross margin account
3. Withdrawal supports cancelling
4. Futures positions supports updating cross margin leverage
5. Add interest in spot margin account response

1. Add a new GateApiException which helps retrieving error label(if the error has one)
2. Fix incorrect order type in list personal futures trades

5.15.2 includes support with all new APIs added since 4.14.0, along with some breaking changes:

1. okhttp is upgraded to okhttp3
2. The authentication process is rewritten. For how to initialize your APIv4 key and secret, refer to any private API endpoint in API documentation
3. Optional parameter passing is changed. In versions <= 4.14.0, all optional parameters are needed even if you don't want to set them.
   Since version 5.15.2, parameters builder is added so that optional parameters can be skipped. For example:

// version <= 4.14.0
futuresApi.listFuturesCandlesticks(settle, contract, null, null, null, null);
// version >= 5.15.2
// note the usage of .execute()
futuresApi.listFuturesCandlesticks(settle, contract).limit(10).execute();