fix(idtoken): provide default scope for cert endpoint (#1198)
fix(idtoken): provide default scope for cert endpoint (#1198)

When NewValidator is called without any options passed in it will
fail talking to the google cert endpoint because the dailed
authenticated client will not have proper scopes and leads to the
error: "invalid_scope". We should set a default scope so this method
can be called with no extra options.

Fixes: #1187
googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client