Consider using new Envoy v1.21.0 features

[sunjayBhatia]

See release notes for new features in Envoy 1.21.0: https://www.envoyproxy.io/docs/envoy/v1.21.0/version_history/current#new-features

Relevant new features include new access log configurations, access log command operators, and http dynamic custom header options.

We should consider which of these new features are interesting to expose and implement them. (e.g. new access log command operators should be allowed since we validate them)

[skriss]

@sunjayBhatia does #4627 catch us up here, or are there more options we should add from 1.21?

[sunjayBhatia]

ah missed this one, not sure we need to block 1.22 on it since people have added the few ones they have needed so not a big signal we're desperately missing something

tbh we should/could do a pass through some of the previous Envoy releases and see what we may have missed exposing, esp. in the access log operator sphere (mostly so we're up to date and don't have to do more maintenance fixes later)

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