Programmatic CCA task/session creation

Create a new session with customized skill/agent/mcp with CCA.
 
Using /delegate, we can just send the prompt to CCA with prompt and customized skills/agent/mcp settings, like used the API below:

```
public async Task<CopilotSession> CreateSessionAsync(SessionConfig? config = null, CancellationToken cancellationToken = default)
```