network auth filter example

*Description*:
> There is an http filter ext auth example, but not a network auth filter one.

The docs do have a partial [example](https://www.envoyproxy.io/docs/envoy/latest/configuration/listeners/network_filters/ext_authz_filter#config-network-filters-ext-authz). However it's not clear to me whether I can use this before a tcp proxy or just the http router.

Start with the linked network ext_authz filter documentation and compare it with the existing HTTP filter example; determine whether the filter can precede a TCP proxy or only an HTTP router. Done means the docs include a complete network auth filter example and explicitly state the supported filter placement.

Written by the indexing model from the issue text.