Promote FieldNameValidator warnings to errors

[blakehurlburt]

Motivation

Currently FieldName and FieldNameValidator warn you when adding invalid field names. One frequent issue is developers copying snake_case naming conventions from other places, not realizing this is supported for legacy endpoints only, and not seeing the warnings.

Proposal

We should promote these warnings to errors if we really want to migrate off this naming convention in the long term. Probably need to do it behind a feature flag for now, so that individual projects have an escape hatch when needed. Also would be good to document the semantics for whether fixing this is an API break and/or an ABI break, so devs can decide whether it's possible to switch or not.