Support Futures Spread Trading

[AlexCatarino]

Expected Behavior

Like options, if we place orders for two futures' contracts to create a spread, the margin will be reduced. There are 3 cases, according to CME:

• INTRAMARKET SPREADS (same future)
• INTERMARKET SPREADS (different but related future)
• COMMODITY PRODUCT SPREADS (same commodity: Soybean Meal and Soybean Oil)

Actual Behavior

Buying power for futures accounts legs independently.

Potential Solution

Implement Position buying power for futures.

Checklist

• [x] I have completely filled out this template
• [x] I have confirmed that this issue exists on the current master branch
• [x] I have confirmed that this is not a duplicate issue by searching issues
• [x] I have provided detailed steps to reproduce the issue