Method to know if a service server on ROS side is up and ready

[ynyBonfennil]

Is your feature request related to a problem? Please describe.
There doesn't seem to be a method in ROS TCP Endpoint / Connector to check if a service server on ROS side is ready to use so I can send a request from unity side properly.

Describe the solution you'd like

• Add a method equivalent to wait_for_service of rclpy
• Or add a method to know if a service server on ROS side is ready or not from Unity side
• Or add a method to list service servers on ROS side

Describe alternatives you've considered

• Send requests to service server without knowing if it's ready or not.
• Create a new publisher on ROS side to notify unity side that a service server is up and ready.

Additional context
I'm using ROS2 Galactic, and I don't understand enough about the compatibility with ROS1.