[refine] Design Server States API

[mdabrowski1990]

Description

As a user, I would like to have functionality of internal server state, so I would be able to simulate complex server ECUs that act the same way as the real one.
As a developer, I would like to have AbstractServerState (or similar) class defined (with its API), so I know the methods and attributes that would be implemented.

Desired state functionalities must be taked into consideration:

• state has predefined list of possible values that it might take
• there is exactly one state value at the moment
• states might depend on each other (change of State1 value might result in change of State2 value), therefore - observer would probably be required
• the state object always stores the actual value (the value is not determined upon a call, but updated without any user interaction)

Acceptance Criteria

• API of Abstract Server State class is defined
• there is no (or little code) - preferably only exceptions (e.g. NotImplemntedError) are raised
• all required functionalities are satisfied by the designed API

[mdabrowski1990]

Testing:

• Integration Tests (example simulation)