Waitset example with UnixDatagramSocket

## Brief feature description

The iceoryx2 Waitset is much more powerful than the one from iceoryx1 since it can also wait on file descriptors. The event multiplexing example should be extended to show this capability.

Locate the event multiplexing example and read the Waitset and UnixDatagramSocket APIs it uses. Extend the example to demonstrate waiting on a Unix datagram socket file descriptor, and verify that the example builds and shows the Waitset handling that event.

Written by the indexing model from the issue text.