Storage: Add internal constructors for output only model types

[tg-msft]

@KrzysztofCwalina pointed out in the ongoing API review that model types only ever returned shouldn't have public constructors. We're not generating those constructors, but C# will add a default public constructor if we don't have any. Here's an example comment.

We need to add an internal constructor that looks something like:

    /// <summary>
    /// Prevent direct instantiation of ${naming.type(type.name)} instances.
    /// You can use ${factoryName}.${naming.type(type.name)} instead.
    /// </summary>
    internal ${naming.type(type.name)}() { }

This is a little more involved. You can see where we generate constructors at generator.ts line 872. We want to add an else that generates the above code. You can get the factory name for the helpful comment via same code as line 943. Adding a local variable for naming.type(type.name) probably wouldn't hurt either.

When you regenerate, you should see private constructors appear for all the model types without other constructors and a Azure.Storage.sln should rebuild cleanly.

[ShivangiReja]

Closed with #7744