Feature request

Feature description

Meta ticket to track improvements we want to make to services and service-related features. This is not an exhaustive list, feel free to complete it

• create a "graph_listener" equivalent to use for wait_for_service (related to Asyncronous wait_for_service #58)
• refactor client to store pending requests
• refactor clients to return a future for each request sent and provide an equivalent of rclcpp::spin_until_future_complete() at the executor level

connects to #170