Trading API: Positions

[haxdds]

Implement routes found in the Trading API positions docs.

def get_positions(self) -> List[Position]:
   pass

def get_position_by_symbol(self, symbol: str) -> Position:
   pass

def close_all_positions(self, request: CloseAllPositionsRequest) -> None:
   pass

def close_position_by_symbol(self, symbol: str, request: ClosePositionRequest) -> None:
   pass