- Added `RealtimeAnnotations` class to manage annotation creation, deletion, and subscription on realtime channels.
- Introduced `Annotation` and `AnnotationAction` types to encapsulate annotation details and actions.
- Extended flags to include `ANNOTATION_PUBLISH` and `ANNOTATION_SUBSCRIBE`.
- Refactored data encoding logic into `ably.util.encoding`.
- Integrated annotation handling into `RealtimeChannel` and `RestChannel`.

- Added `RealtimeAnnotations` class to manage annotation creation, deletion, and subscription on realtime channels.
- Introduced `Annotation` and `AnnotationAction` types to encapsulate annotation details and actions.
- Extended flags to include `ANNOTATION_PUBLISH` and `ANNOTATION_SUBSCRIBE`.
- Refactored data encoding logic into `ably.util.encoding`.
- Integrated annotation handling into `RealtimeChannel` and `RestChannel`.

… handling

- Refactored to mandate the `Annotation` type across annotation-related methods in `RealtimeAnnotations` and `RestAnnotations`.
- Introduced `_copy_with` in `Annotation` for simplified object cloning with modifications.
- Enhanced data validation in `encode_data` to raise `AblyException` for unsupported payloads.

- Added support for updating annotation fields (`id`, `connectionId`, `timestamp`) from protocol messages.
- Introduced validation for required annotation fields in `RestAnnotations`.
- Enabled idempotent annotation publishing with auto-generated IDs.
- Improved error handling for annotation processing in `RealtimeChannel`.
- Allowed unsubscribing all annotation listeners when no arguments are provided.

- Refactor publish/delete to use shared __send_annotation() with explicit
  action setting per RSAN1c1/RSAN2a/RTAN1a/RTAN2a
- RTAN4e: Change subscribe mode check from exception to warning per spec;
  guard against empty modes when server doesn't send flags
- RTAN4c/RTAN5a: Support array of types in subscribe/unsubscribe
- RSAN1c4: Fix idempotent ID generation to use base64(9 random bytes):0
- Export Annotation, AnnotationAction, ChannelMode, ChannelOptions from ably
- Use isinstance() consistently for bool checks across channel modules

[AIT-316] feat: introduce support for message annotations

Release/3.1.0