Feature request: add support perps

[CRossel87a]

Hi,

Nice crate!

I was able to add perp support by adding an optional leverage field to the create order struct. The default leverage is 1.0 if left blank. This causes insufficient margin error.

Pulling perp portfolio also causes json deserialize error, but have not looked into the details.

[Ohkthx]

Hello! I'll have to check the API documentation.

The main problem for me adding support for perps/futures would be that I have no way to really test it works correctly. Maybe the sandbox supports the endpoints now but I am not enrolled to do those on production.

I can try and see why the portfolios don't work. A lot of times the API documentation will say it returns a field as a string but in reality it is an integer or float (or vice versa).

The perp portfolio fix would be great for the current 2.0.2 branch that I am slowly working on.