[Core] Add warning on headers with underscores

Created by: ybelenko

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Closes #4217 (closed)

I didn't add any tests, because underscore search is pretty basic in current PR. I can add tests if necessary.

Examples of new warnings:

[main] WARN  o.o.codegen.DefaultCodegen - Security schema 'api_key' mapped to 
underscored header 'api_key' which is forbidden in Apache and Nginx webservers 
by default. 
https://stackoverflow.com/questions/22856136/why-http-servers-forbid-underscores-in-http-header-names/22856867#22856867

[main] WARN  o.o.codegen.DefaultCodegen - Request header parameter name 
'enum_header_string_array' contains underscore which is forbidden in Apache 
and Nginx webservers by default. 
https://stackoverflow.com/questions/22856136/why-http-servers-forbid-underscores-in-http-header-names/22856867#22856867

[main] WARN  o.o.codegen.DefaultCodegen - Response header name 'X_Rate_Limit' 
contains underscore which is forbidden in Apache and Nginx webservers by 
default. 
https://stackoverflow.com/questions/22856136/why-http-servers-forbid-underscores-in-http-header-names/22856867#22856867

cc @OpenAPITools/generator-core-team