v0.13.0

• add Sync binary library variants for all supported platforms
• add MacOS universal binary library, supporting Intel x64 and Apple Silicon arm64
• split Sync symbols out of objectbox.h/pp into objectbox-sync.h/pp
• add Sync server-time getter, listener and local-to-server diff info - to access server time info on clients
• add Sync heartbeat interval configuration and an option to send one immediately
• semi-internal: update Dart/Flutter SDK to v2.0