Improve CI and WORKSPACE management in envoy-filter-example

Following up from https://github.com/envoyproxy/envoy-filter-example/pull/62. A few systematic issues we should think about fixing in this repo:

1. I'm not seeing the .circleci/config.yaml build/test phases running as part of the GH checks here.
2. It would be nice to get regular nag e-mails from CircleCI when this repo is broken, rather than waiting for folks to discover this and file issues.
3. We should probably push WORKSPACE directly from envoyproxy, similar to what we do for data-plane-api.