When running `nimbus_light_client`, we persist the latest header from
`LightClientStore.finalized_header` in a database across restarts.
Because the data format is derived from the latest `LightClientStore`,
this could lead to data being persisted in pre-release formats.
To enable us to test later `LightClientStore` versions on devnets,
transition to a `ForkedLightClientStore` internally that is only
migrated to newer forks on-demand (instead of starting at latest).