C++ Guidelines - Add a concept of the ::Models namespace where response types need to live

[ahsonkhan]

You can look at .NET's guidelines as an example:

https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-service-models-namespace

YOU MAY place output model types in .Models subnamespace to avoid cluttering the main namespace with too many types.
It is important for the main namespace of a client library to be clutter free.

cc @RickWinter, @CaseyCarter, @tg-msft

[github-actions]

Hi @ahsonkhan, we deeply appreciate your input into this project. Regrettably, this issue has remained inactive for over 2 years, leading us to the decision to close it. We've implemented this policy to maintain the relevance of our issue queue and facilitate easier navigation for new contributors. If you still believe this topic requires attention, please feel free to create a new issue, referencing this one. Thank you for your understanding and ongoing support.