Add two example packages of service client and service server to understand asynchronism

[santiago-tapia]

This PR add two new packages in C++ services examples.

• The server example introduces a response delay in the service, configurable via a parameter. The purpose of this example is to bring the service response time to a user-perceivable scale, allowing the effects of such delays to be better observed.
• The client example uses a callback to receive the server's response. By using a callback, the main program of the node can follow the usual implementation of the main function in any other node; the spin method can be used, and there is no synchronous blocking call to wait for the response.

[MichaelOrlov]

@santiago-tapia Thanks for the PR.
We have discussed this PR on today's issue triage meeting, and we are curious how these proposed examples differ from the existent services tutorial https://docs.ros.org/en/rolling/Tutorials/Beginner-Client-Libraries/Writing-A-Simple-Cpp-Service-And-Client.html

We think that It would be better to change the existing tutorial rather than add new examples.