Add de-duplication for websockets

[juan-campuzano]

Description

This pull request introduces deduplication support for WebSocket (WS) and Server-Sent Events (SSE) transports in the Dart client, along with some improvements to the example app and backend API. The deduplication logic prevents processing duplicate messages by maintaining a capped cache of message IDs, which can be configured or disabled via the new options in AsyncConfig.

Category

• Feature
• Fix

Checklist

• The pull request is complete according to the guide of contributing
• Automated tests are written
• The documentation is up-to-date
• the version of the mix.exs was increased
• The pull request has a descriptive title that describes what has changed, and provides enough context for the changelog

[gabheadz]

Thanks!