New Invoke demo in C#

[bernardnormier]

This PR implements a new Invoke demo in C# (Invoke2 for now, until we retire invoke).

I initially wanted to call it "SliceFree" and provide a Slice-free demo. However, even though this demo does not provide any Slice definition or use any generated code, it still uses InputStream / OutputStream to marshal/unmarshal data (strings) using the Ice encoding.

Using InputStream / OutputStream is unavoidable with Ice, even if we just want to transfer bytes.

This demo is primarily about demonstrating ice_invokeAsync (on Ice.ObjectPrx) and dispatchAsync (on Ice.Object).
In practice, these methods are more useful to build a Forwarding servant than to send Slice requests "by hand". Maybe we should drop this Invoke demo and provide a Forwarder demo instead?

[pepone]

In practice, these methods are more useful to build a Forwarding servant than to send Slice requests "by hand". Maybe we should drop this Invoke demo and provide a Forwarder demo instead?

Yes. I think a Forwarder demo makes more sense. Create Ice requests using the OutpuStream API is not a common use case.