[http-client-csharp] Introduce a mechanism to disable a visitor from dependencies

[ArcturusZhang]

now Mgmt Generator as a dependency, it gets the visitors in MTG and Azure Generator.
Something is different in DPG and MPG, therefore there are some visitors we would not need.
Could we introduce a mechanism to disable a inherited visitor?

[ArcturusZhang]

Since the issue that drove us to require this feature has been resolved by a recent update, this should not have high priority. But I think this kind of flexibility is nice to have.