Add Response abstraction

[m-nash]

We will need an abstraction for a response from a client service call in order to be able to replace this via plugins.

Methods that need to be provided include (not exhaustive)

• Type
• GetRawResponse
• IsError
• Buffering
• Failure exception
• derived types such as
  • ErrorResponse
  • NullableResponse

[ArcturusZhang]

So I think maybe this could something like:

public abstract class ClientResponse
{
	public CSharpType Type {get;}
	public CSharpType ExceptionType {get;}
	public CSharpType ErrorVariantType {get;}
	public CSharpType NullableVariantType {get;}
}

public static class ClientResponseSnippets
{
	public ValueExpression GetRawResponse(this ScopeApi<ClientResponse> response) {}
	// etc
}

so that we separate this thing into two parts: data (containing the types we use), and actions (containing the methods/properties we could call)