Allows users to specify 'nonPublicApi' additional property (and C#
client CLI switch) to reduce visibility of classes created by the
generator. This includes API and Models as well as supporting code like
ApiClient and other infrastructure.
The requirement is to support codegen generated code to be embedded
within other applications where the generated code is not intended to be
publicly consumable or publicy exposed. An example would be an SDK which
internally consumes an API via the generated code; we wouldn't want the
internal API implementation exposed as part of that SDK.
Reducing visibility of the classes effectively makes the entire
implementation internal, regardless of the public modifier on methods or
static members. To fully make all members internal it would require
explicit interface implementation, which is not ideal.
see #4401
