[http-client-csharp] `CodeGenType` improvement suggestion

[ArcturusZhang]

We have scenarios that we need to change namespace of enum types, but enum types do not support partial, therefore we will have to move all the content into the customization, which makes it no longer to be able to catch up new updates.
Could we somehow update this to support this scenario?
For instance, we could add a new attribute like this:

[CodeGenNamespace("TheTypeName"]
namespace NewNamespace;

Or we could find other ways.

[JoshLove-msft]

Could we also use @clientNamespace decorator instead of doing the customization in C# code?