Refactored stop signal handling in all components to use `tokio_util::sync::CancellationToken` instead of `tokio::sync::watch`.
- Improved cancellation logic by leveraging `CancellationToken` for cleaner and more efficient handling.
- Updated corresponding dependency to `tokio-util` version `0.7.14`.
- Split `verify-era-proof-attestation` into modular subcomponents for maintainability.
- Moved client, proof handling, and core types into dedicated modules.