Update, and add testing for the envoy-filter-example and/or make into a sandbox

[phlax]

Currently we use the envoy-filter-example in CI but i think only as an external workspace to test building from (im guessing that was why it was set that way)

It seems the actual example is not getting tested and possibly doesnt work as expected

We could probably make it into a sandbox and add testing/documentation to ensure it stays working

[shiponcs]

Adding this https://envoyproxy.slack.com/archives/C78HA81DH/p1690280066893899 so that people can later understand what triggers this idea.

[shiponcs]

Here is a related issue: envoyproxy/envoy-filter-example#178

[github-actions]

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