Add support for AG-UI Protocol

[flolas]

Is your feature request related to a problem?
No

Describe the solution you'd like
I would like to see the required serialization and deserialization logic implemented for the AG-UI Protocol. https://docs.ag-ui.com/introduction

Additional context
Several LLM Frameworks got ag-ui support: https://ai.pydantic.dev/ui/ag-ui/, https://google.github.io/adk-docs/tools/third-party/ag-ui/, https://pypi.org/project/ag-ui-langgraph/,