Refactor ConversationStore to follow SettingsStore pattern

[tofarr]

This PR refactors the ConversationStore class to follow the same pattern as SettingsStore:

• Make ConversationStore an abstract base class
• Create FileConversationStore implementation
• Move ConversationMetadata to server.data_models
• Reorganize storage package structure

All usages of ConversationStore.get_instance have been updated to use the implementation and supply a token similar to SettingsStoreImpl.get_instance.

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:2fc946f-nikolaik   --name openhands-app-2fc946f   docker.all-hands.dev/all-hands-ai/openhands:2fc946f