[http-client-csharp] Consider to introduce a hierarchy structure for namespace

[ArcturusZhang]

In our generator, we need to do some escaping on namespace names, for instance in clients, we might need to add underscore to namespace segments when there is a name collision.
Now we are doing this in ClientProvider - but now, we are using string to represent the namespaces, therefore when we change the namespace, it will not take effect on all the namespaces across our library.

If we could change the namespace to be represented by an object with hierarchical structure, we could modify it freely without having to maintain those strings manually.

[ArcturusZhang]

I found a workaround for this maybe we could defer the discussion