[.Net][Feature Request]: Add a specific message type for function middleware message

[LittleLittleCloud]

Is your feature request related to a problem? Please describe.

Currently, the return message type from function middleware is AggregateMessage<ToolCallMessage, ToolCallResultMessage>, which is too long and looks nasty. We should come up with a specific message type to replace it because function middleware is so widely used in AutoGen.Net

Describe the solution you'd like

public class FunctionCallMiddlewareMessage: AggregateMessage<ToolCallMessage, ToolCallResultMessage>

Additional context

No response