Expose a way to check if the client has a working connection to the server

[rainerschoe]

The motivation is that users want to implement Fallback services (e.g. a different AppConfig server instance / region) in case a connection failure persists.

We could in theory extend the FallbackDataSource meachanism we already have. However the Node / Python / Go clients have a way simpler implementation:

They expose a isConnected() method and all fallback logic is then responsibility of the user.