Add hasPrefix and hasSuffix template funcs

[maks56893]

I'm trying use custom generate templates and want to add some methods for only response models. For define what is request or response I have suffixes for models. But for now I can't define it in template

[jamietanna]

Would be happy to review a PR to add them to the TemplateFunctions

(Although needs better documenting in #1487, an alternate path for now is to add them to call oapi-codegen as a library, where you can modify this in your own code)

[maks56893]

here is PR