DAL: Health RPC should also include information on whether the DAL node is synced wrt to L1

Currently, the health check RPC for the DAL node does not report if the node is still syncing its state with its corresponding L1 node. This lack of information makes it difficult for users and applications to know whether the DAL node is fully caught up.

We propose to either extend the existing /health RPC or create a new /synced endpoint, similar to the L1 /bootstrapped endpoint. A dedicated /synced endpoint could be designed as a streamed or blocking RPC that only returns once the node is fully synchronized with L1. This change would help improve monitoring and reliability of the DAL node's state.