Guarantee and optimize v3 internally until close to v4alpha

Given that v4 might be further out than the API clock dictates today (https://github.com/envoyproxy/envoy/issues/10852), it might be beneficial to work with v3 inside of Envoy rather than bump to v4alpha. This would allow us to skip `VersionConverter::upgrade` for v3 protos in `onConfigUpdate()`, eliminating a source of non-trivial overhead that exists today.

This issue will track discussion around using v3 vs. v4alpha and future optimization work to skip the upgrade for known latest version protos (should be very easy to bypass based solely on type URL analysis).