regexgame-backend: Add game configuration

[WuestFlorian]

Add a game configuration object to the backend, which includes all metadata about a configured game.

Saved Metadata:

• id (generated)
• allowed regex structures for random generation
• starting complexity factor (target regex character length, or regex charset size)
• whether the complexity should increase with each challenge
• amount of generated answers

DoD

• [x] A game configuration object exists
• [x] A accompianing game configuration DTO exits
• [x] The game configurations are saved in a database