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.