[Core] Add warning on GET request with body

[ybelenko]

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Closes #4216

Warning looks like:

[main] WARN  o.o.codegen.DefaultCodegen - DELETE operation 'deleteOrder' contains body 
parameter. Please, make sure your framework handles DELETE method with body properly.

[main] WARN  o.o.codegen.DefaultCodegen - GET operation 'testEnumParameters' contains 
form parameter. Please, make sure your framework handles GET method with form properly.

cc @OpenAPITools/generator-core-team

[wing328]

Thanks for the PR. Similar feedback as #4412 (comment)

[jimschubert]

Included in #4979