[Feature]: AG-UI protocol should be extensible

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

If servers add extra data to AG-UI events, the Zod schema should not filter it.

In my case AssistantMessages (and other messages) to have a timestamp for display in the UI.

### Proposed Solution

It should use a passthrough rather than strip policy, to allow applications to layer on additional semantics onto the messages.

### Alternatives Considered

All sorts of hacks.

### Additional Context

_No response_