Support Application Default Credentials

[luke-certifyos]

I'd like to be able to use my application_default_credentals.json instead of a service account. (I don't have iam.serviceAccounts.list permissions)

Right now I'd get the error:

Connection test failed: java.lang.ClassCastException: class com.google.auth.oauth2.UserCredentials cannot be cast to class com.google.auth.oauth2.ServiceAccountCredentials (com.google.auth.oauth2 .UserCredentials and com.google.auth.oauth2.ServiceAccountCredentials are in unnamed module of loader com.intellij.ide.plugins.cl.PluginClassLoader @5def47aa)