Allow to disable lowercase snake_case warning for fields

Pull request #6005 introduced a change that unconditionally prints warnings when fields do not follow the `snake_case` naming convention using lowercase letters. While this fix was implemented in good faith, it poses unnecessary and unacceptable risks: Projects may chose to not adopt this naming convention for compatibility, resource, risk mitigation, or even unknown reason. If these fields are not corrected, actual problems could be masked by the immense number of build log messages generated by this warning.

Concerns about this change were raised in the past, but got ignored: I've found #6032, #7111, #8612, there might be more.

The patches contributed with this bug report try to address this issue by fine-tuning the scope of the existing `--no-warnings` command line option, and by automatically adoption the developer's intention expressed via the `--cpp-field-case-style` flag.

Start by locating the command-line handling for --no-warnings and --cpp-field-case-style, then reproduce the lowercase snake_case warning for a nonconforming field. Done means projects can suppress or avoid this warning according to those options without masking unrelated build warnings; add or update coverage for both flag behaviors.

Written by the indexing model from the issue text.