Use FindDefaultCredentials and a local ADC when the metadata server isn't available

I've been removing unnecessary dependencies on exported Service Account keys (see https://github.com/google/osv.dev/pull/822 for context), which works as desired using Cloud Build's default Service Account when running under Cloud Build.

When running locally, things go south at https://github.com/GoogleCloudPlatform/esp-v2/blob/5c4538a5a46f34e30fca009cd190934fd366ceee/src/go/configmanager/config_manager.go#L137 though because of the lack of a metadata server.

Someone who knows more about Cloud Identity matters than me pointed me at https://pkg.go.dev/golang.org/x/oauth2/google#FindDefaultCredentials, which by the looks of it would fall back to a local ADC in the event of the metadata server not being available.