[http-client-csharp] It is incorrect that OAuth2 authentication defined in the spec is silently ignored

[ArcturusZhang]

Currently we do not support OAuth2 in unbranded client gen right now, and the current behavior in our code gen is to ignore it as if there is no auth (when only OAuth2 is defined).
This behavior is kind of incorrect - we generate wrong code and the user never gets a notification.

It also does not make sense if we throw exception whenever an OAuth2 authentication is defined - the user will never get a valid result or even a partial result so that they could make customization code to support such scenarios.

One possible solution for this is that we write a warning for this to inform the user that this is currently unsupported.

[JoshLove-msft]

Closing as this was fixed - we report a warning if OAuth is defined in the spec.