Pure static classes to namespaces

[capoferro]

Commit Message: Reviewer prefers namespaces to static classes (Envoy style guide doesn't mention this).
Additional Description: This is a followup from comments in #17728
Risk Level: Very Low
Testing: Existing

[capoferro]

/retest

[repokitteh-read-only]

Retrying Azure Pipelines:
Retried failed jobs in: envoy-presubmit

🐱

Caused by: a #17812 (comment) was created by @capoferro.

see: more, trace.

[jmarantz]

"Envoy prefers namespaces to static classes." --> citation needed :)

I can't find any reference to that in https://github.com/envoyproxy/envoy/blob/main/STYLE.md -- and I thought it was plenty common to use statics in a class in Envoy.

[capoferro]

"Envoy prefers namespaces to static classes." --> citation needed :)

Oh... I got a comment on the last PR that I should use a namespace instead. I don't personally have a preference.

#17728 (comment)

[jmarantz]

Heh -- I personally prefer static class methods for various reasons, but don't feel that strongly. I'll defer to @dmitri-d :)

[dmitri-d]

Haha, I don't actually feel strongly about this either, I thought that was the preferred way for grouping of static functions in Envoy. Lgtm.

[capoferro]

@jmarantz Are you the one to hit the merge button?

[jmarantz]

I'm OK merging this but IMO this is not at all preferred by Envoy style, for the record. I think Envoy is OK with either one and there are a ton of existing uses of classes with static members, and there are some benefits to that approach.