dev: Easier development in sourcegraph.com mode
Created by: ryanslade
When running with SOURCEGRAPHDOTCOM_MODE=true set we require that at least one GitHub and GitLab external service have their cloud_default flags set.
This change allows us to enforce this by setting the CloudDefault field on config in dev-private.
Part of: https://github.com/sourcegraph/sourcegraph/issues/18151
Associated update to dev-private: https://github.com/sourcegraph/dev-private/pull/25