Inform client if ContentHub service is not available

An app using the Content Hub QML API might have the need to know whether service is actually available, in order to fall back to other means if it isn't.

This is especially relevant for snaps: if the snap uses the content from ubuntu-app-platform, the QML module of the ContentHub will be available, but this doesn't guarantee that the CH service is running. It might not be installed (for example, in another distro), not running or maybe not reachable due to confinement.

I propose adding a "valid" property to the ContentHub singleton which would tell whether the service is available.