Make google credentials debugable

[jroper]

The GoogleSettings class in Alpakka is very private. It won't tell you anything about what credentials it has loaded, in case you want to log something to debug to make sure that it's loading the credentials you expect it to load. It would be nice if it could provide some way of interrogating it, or at least logging, what the name of the service account that it had located is, for example.

[proxo]

what's more order of parsing credentials when provider = application-default may cause issues when running the same code & config on different clouds i.e azure vs gcp. The config parsing should be more explicit or visible in logs.