refactor: Pydantic models for Game State

[S1M0N38]

For the moment, we are not really using the Game State (G) pydantic models (just in some tests).

• Do we need them?
• Could they provide a good way to document game state?
• Is it better to use Lua types?

[S1M0N38]

Maybe having well defined Pydantic models could be useful for writing validation tests against game state (G) returned by lua API (instead of the verbose https://github.com/S1M0N38/balatrobot/blob/df0830db2fb9abed89d3d9d1158fb27b8be9953b/tests/lua/test_api_functions.py#L731-L773 )