Is it possible to store data types in the Azure app configuration? Or is it left to the program calling the app configuration to declare the data type?
Absolutely. Each key-value in App Configuration has a content-type field. While you can put anything in it as long as your application understands it, we do recommend using MIME types, which is also what the out-of-box data types App Configuration uses.
More discussion can be found at https://github.com/Azure/AppConfiguration/issues/116#issuecomment-559628997