[FEAT] Add Support for Flutter

Summary

Add a Flutter SDK for Tambo to enable mobile developers to build AI-powered Flutter apps.

[!IMPORTANT] This issue is to gauge interest only. We are not looking for contributors on this issue. If you have proven experience in developing new SDKs and integrations for Flutter, we encourage you to make an example/community sdk in your own repository. If you have suggestions or questions, please reach out on our discord.

Motivation

Flutter is one of the most popular cross-platform mobile frameworks. Supporting Flutter would expand Tambo's reach to mobile developers.

Scope

• Flutter package for Tambo integration
• Dart types mirroring the TypeScript SDK
• Documentation and examples