refactor: Pulled business-logic into class `ShardedChatHome`

refactor: Pulled business-logic into class `ShardedChatHome`

- Pulled the logic that selects the appropriate shard from the class
  `ChatBackendController` into the newly introduced class
  `ShardedChatHome`.
- Simplified the configuration
  - `InMemoryServicesConfiguration` creates a `ChatHome` of type
    `SimpleChatHome`, if the `sharding-strategy` `none` is choosen.
  - In that case, the values for `num-shards` and `owned-shards` are
    ignored and set to `1` and `0`.
  - If the `sharding-strategy` is set to `kafkalike`, a `ChatHome` of
    type `ShardedChatHome` is instanciated and the configuration
    respects the configured sharding.
- Simplified the configuration of `ChatBackendControllerTest` accordingly.