GameCore/Backend first version implementation

[henrysneps]

(this issue is currently in an invalid state, and its scope needs to be redefined)

First version of the Game Core -component where it is able to handle all requests without throwing any exceptions. Partial gameplay logic will be implemented at this stage. Interface for the GameCore will be developed dynamically depending on the surfacing needs of fronted.

Preliminary work:

• [ ] Find an unit test framework which is independent from Unity
• [ ] Verify the use of said unit test framework

Implementation (sketch):

• [ ] Basic functionality
• [ ] Advanced functionality
• [ ] Interface

[henrysneps]

Old class diagram sketch to be iterated on.