Add callback model

Created by: devplayer0

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

This adds a new CodegenCallback class, a list of which is now present in CodegenOperation. CodegenOperation now also includes a isCallbackRequest boolean since fromCallback() (the method added to DefaultCodegen to process callbacks in OpenAPI operations) uses CodegenOperation as the model for a callback request.

A CodegenOperation which represents a callback request will have a null operation id.

A test is included for this new model.

(See #372)