Make envoyproxy/envoy the source-of-truth for envoy-filter-example

As discussed in https://github.com/envoyproxy/envoy/pull/5031#issuecomment-438757841, we have a multiple commit dance needed for changes that break the filter API today, where both envoyproxy/envoy and envoyproxy/envoy-filter-example need multiple commits. One way to avoid this would be:

1. We move envoy-filter-example into the main Envoy repo, using a local repository in the WORKSPACE to point the example at Envoy source.
2. We export to envoy-filter-example as a read-only mirror (like data-plane-api), and swizzle the WORKSPACE to one that uses a remote repository.

This preserves envoy-filter-example as a standalone example of integrating Envoy, while avoiding the multi-phase commit dance (and breakages) we hit regularly these days.