v0.61.0

- Net: Chat now throws RateLimitedError/RetryLaterException when the server requests a retry
- Node: Switched from yarn to npm to align with other Signal projects
- Android: Split ChatService into Unauthenticated.. and AuthenticatedChatService for platform consistency
- Attest: Removed unused IAS attestation code and tests
- Backup: Made timestamps optional for AttachmentLocator:uploadTimestamp and ChatItem:dateServerSent