Gate-ws

- update futures models, fix fields wrong type
- add futures model `FuturesPositions`

- Support websocket skip tls verify with `SkipTlsVerify` of ConfOptions
- Update futures models
- Update examples

- Add constant `ChannelSpotCrossBalance` support `spot.cross_balances` channel
- SpotUserTradesMsg add field `Text` for orders' text

python-0.3.0
- Support cross margin balance update

- fix futures book ticker and order book update struct

- fix futures order book update struct

- add futures order book struct