Feature request

Feature description

Currently service client does not support synchronous send_request, but only asynchronous. thinking of affinity for ROS1 and SyncParametersClient, it would be nice to support sync_send_request.

https://github.com/ros2/demos/blob/11e00ecf7eec25320f950227531119940496d615/demo_nodes_cpp/src/services/add_two_ints_client.cpp#L24, this is actually tagged with TODO

Note

https://discourse.ros.org/t/my-ros2-minimal-examples-difficulties-coming-from-ros1/18705