Implement unit tests for C++

[RobPasMue]

At the moment, we have several functional tests which are not run as part of the CI job. These tests are basically compiled in each of the projects: C++ REST Server, C++ REST Client, C++ gRPC Server, C++ gRPC Client.

In order to run those tests one has to run the binaries simultaneously (i.e. deploy the server and run the client)... This should be automatized in CI using services for deploying the server docker containers and running the client binaries.

An even better approach would be to actually implement unit tests using the Google Test infrastructure. And run them with ctest. However, since they are different projects each of them, it may be necessary to set up mockups in each of them to replicate the behaviour of the server in the client project, and vice versa. TBDiscussed this last approach.