Add sample for `ChatMessageManager`

[MartinZikmund]

What would you like to be added:

Uno Gallery sample on using ChatMessageManager API . See

• https://platform.uno/docs/articles/features/windows-applicationmodel-chat.html

Why is this needed:

Developer samples

For which Platform:

All

[dbarnwal]

can i work on this ?

[morning4coffe-dev]

@dbarnwal Sure! If you have any questions along the way just let me know