Order and Execution Management

Design an `Order` object to represent market and limit orders for single and multi-leg strategies.

Implement a function to construct a multi-leg combo order for a credit spread.

Build an `ExecutionHandler` to submit, modify, and cancel orders through the broker API.

Implement logic to handle order status updates (submitted, filled, cancelled) from the broker.

Develop a position synchronizer to reconcile internal position state with the broker's records.