[Feature]: Support for Mastra workflows

### Pre-flight Checklist

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

### Problem or Motivation

You can't current use CopilotKit or AG-UI with Mastra workflows only with agents.

### Proposed Solution

Add support for Mastra Workflows. Those take a structured JSON as input instead of a message history, and have special data items as part of the output stream that will require more specialized rendering for workflow steps.

### Alternatives Considered

Use something else.

### Additional Context

_No response_