[REQ] Add warning on headers with underscores

[ybelenko]

Is your feature request related to a problem? Please describe.

Already discussed a bit in #3658. Quote:

Apache and Nginx webservers avoid underscores in HTTP headers, so api_key header received as api-key. Refs: Cannot retrieve all request headers using PHP slim framework and Why underscores are forbidden in HTTP header names.

Describe the solution you'd like

Maybe we should add warnings when headers contains underscores. I would add this to DefaultCodegen. I've read somewhere that headers with underscores can cause server security problems.

Describe alternatives you've considered

We can add generator option to disable this warnings. This might be useful when user already configured Nginx/Apache to allow headers with underscores.

Additional context

cc @OpenAPITools/generator-core-team

[ybelenko]

Thanks to @jimschubert
Closed via #4979