feat: Support position intent in order requests

[matebudai]

Context

• Accepting position intent in order request
• Feature request

Changes

• trading enums are extended with a class called PositionIntent
• order request accept this as a new optional parameter

  BUY_TO_OPEN = "buy_to_open"
  BUY_TO_CLOSE = "buy_to_close"
  SELL_TO_OPEN = "sell_to_open"
  SELL_TO_CLOSE = "sell_to_close"