Changes to Get Historical Positions

for the query request parameter subaccount, the min was set to 0.00

added the new optional query request parameter subaccount

added the required property market_positions/items/exchange_index to the response with the 200 status

removed the optional property service from the response with the 400 status

removed the optional property service from the response with the 401 status

removed the optional property service from the response with the 500 status

endpoint added