Custom API Crentials in gsettings

[Gaming4LifeDE]

For easier testing/usage, I think the gsettings schema should be extended to allow setting the API credentials. I don't know if this should be exposed in the GUI because this can lead to users breaking their clients.

Maybe this will be a good opportunity to introduce developer/debugging settings in general, and have custom API credentials be one of them.

I started this issue based on a conversation with @alissonlauffer here.