Add ExecSignal and PosSizeContext class (#138)

src/pybroker/__init__.py

@@ -21,7 +21,7 @@
     enable_model_cache,
 )
 from pybroker.common import BarData, DataCol, Day, FeeMode, PriceType
-from pybroker.context import ExecContext
+from pybroker.context import ExecContext, ExecSignal, PosSizeContext
 from pybroker.config import StrategyConfig
 from pybroker.data import Alpaca, AlpacaCrypto, YFinance
 from pybroker.eval import EvalMetrics, BootstrapResult