feat(code/example): Add persistence to the example app

[romac]

• Persist undecided blocks in a redb database
• Persist decided blocks in a redb database
• Persist certificates in a redb database

Those can be implemented based on the block store of the Starknet mock app.

Once this is done, we should be able to switch the integration test suite from the Starknet mock app to the example app, and then extract the Starknet mock app into its own repository. See #709