Generate openapi.json for the Execution API sub-app (#43796)

`include_in_schema=False` this flag meant none of the path operations were included in `/execution/openapi.json`. This was initially done to ensure this paths aren't included in the top-level client but since it is a separate App, it doesn't include the path operations on `/openapi.json` (used by UI and generated OpenAPI client) but only for App on `/execution` .

airflow/api_fastapi/execution_api/app.py

@@ -28,7 +28,6 @@ def create_task_execution_api_app(app: FastAPI) -> FastAPI:
     task_exec_api_app = FastAPI(
         title="Airflow Task Execution API",
         description="The private Airflow Task Execution API.",
-        include_in_schema=False,
     )
 
     task_exec_api_app.include_router(execution_api_router)