Adds a very simple User class and methods for creation, retrieval and deletion. This allows us to test the user creation/deletion of the GRPC API.
The User class and the methods are only available when the newly introduced "BUILD_ACM" macro is set. The make files and the pipeline has been updated for this.
The User class is designed after the delegator/wrapper pattern where UserImpl is the delegate. The UserImpl class is a wrapper for protobuf's User message objects. This might look clumsy at first but it has the advantage that the grpc/protobuf code is encapsuled and hidden in the *.cpp file. This is a first test which demonstrates how we can remove the grpc/protobuf dependency from the headers altogether (which will make it easier to compile the library as a shared library on windows).
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