Add already filled quantity to Order instance when PARTIALLY_FILLED

[Egisess]

I propose to add to Order class in self.cache property which reflects how much of this order was already filled:

For example we get these consecutive events:

[1m2025-04-29T00:34:00.029000000Z[0m [INFO] BACKTEST_TRADER-001.MyStrategy: <--[EVT] OrderFilled(instrument_id=SOLUSDT-PERP.BINANCE, client_order_id=O-20250429-003400-001-000-1, venue_order_id=BINANCE-1-001, account_id=BINANCE-001, trade_id=BINANCE-1-001, position_id=SOLUSDT-PERP.BINANCE-MyStrategy-000, order_side=SELL, order_type=MARKET, last_qty=0.04, last_px=148.02 USDT, commission=0.00000000 USDT, liquidity_side=TAKER, ts_event=1745886840029000000)[0m
[1m2025-04-29T00:34:00.029000000Z[0m [1;31m[INFO] BACKTEST_TRADER-001.MyStrategy: order: MarketOrder(SELL 1.00 SOLUSDT-PERP.BINANCE MARKET GTC, status=PARTIALLY_FILLED, client_order_id=O-20250429-003400-001-000-1, venue_order_id=BINANCE-1-001, position_id=SOLUSDT-PERP.BINANCE-MyStrategy-000, tags=['short'])�[0m

Which means that 0.04 of initial order was just filled and the status of Order was changed to PARTIALLY_FILLED - it will be really convenient if we also knew how much of each Order was Filled, and get events like this:

[1m2025-04-29T00:34:00.029000000Z[0m [INFO] BACKTEST_TRADER-001.MyStrategy: <--[EVT] OrderFilled(instrument_id=SOLUSDT-PERP.BINANCE, client_order_id=O-20250429-003400-001-000-1, venue_order_id=BINANCE-1-001, account_id=BINANCE-001, trade_id=BINANCE-1-001, position_id=SOLUSDT-PERP.BINANCE-MyStrategy-000, order_side=SELL, order_type=MARKET, last_qty=0.04, last_px=148.02 USDT, commission=0.00000000 USDT, liquidity_side=TAKER, ts_event=1745886840029000000)[0m
[1m2025-04-29T00:34:00.029000000Z[0m [1;31m[INFO] BACKTEST_TRADER-001.MyStrategy: order: MarketOrder(SELL 1.00 SOLUSDT-PERP.BINANCE MARKET GTC, status=PARTIALLY_FILLED, client_order_id=O-20250429-003400-001-000-1, venue_order_id=BINANCE-1-001, position_id=SOLUSDT-PERP.BINANCE-MyStrategy-000, tags=['short'], filled_qty = 0.04)�[0m

Also this would be logical because our venue has not size = 1 order but size = 0.96 order hanging.

Would like to hear any other proposals on this topic.