New "group_id" and "group_role" fields on orders

Broker API

Adds two optional fields to the Order and OrderLeg schemas so multi-leg orders can be correlated, including on the trade events stream where legs arrive one at a time with legs: null.

• group_id: an opaque UUID shared by every order in one bracket, oto, or oco order, including legs and replacements.
• group_role: a new OrderGroupRole enum with values entry, take_profit, and stop_loss. For oco, the top-level order is take_profit.
• Both keys are omitted for orders that are not part of a bracket, oto, or oco order.
• New examples cover a bracket order response and a bracket-leg trade update event.

Affected Endpoints

• POST /v1/trading/accounts/{account_id}/orders
• GET /v1/trading/accounts/{account_id}/orders
• GET /v1/trading/accounts/{account_id}/orders/{order_id}
• GET /v1/trading/accounts/{account_id}/orders:by_client_order_id
• PATCH /v1/trading/accounts/{account_id}/orders/{order_id}
• DELETE /v1/trading/accounts/{account_id}/orders
• POST /v1/trading/accounts/{account_id}/orders/estimation
• DELETE /v1/trading/accounts/{account_id}/positions
• DELETE /v1/trading/accounts/{account_id}/positions/{symbol_or_asset_id}
• GET /v1/rebalancing/runs
• GET /v1/rebalancing/runs/{run_id}
• GET /v2/events/trades