Gateio futures support

[akasimo]

Description of code - what bug does this fix / what feature does this add?

adding support for gateio futures

• [x] - Tested
• [x] - Changelog updated
• [x] - Tests run and pass
• [x] - Flake8 run and all errors/warnings resolved
• [ ] - Contributors file updated (optional)

[bmoscon]

@akasimo if you can resolve the conflict (which will also resolve my comment) I will merge this