[LLC] [Synapse] Enable cast from Response to a hand-written model

[annelo-msft]

Problem

As we add high-level methods to the Synapse Access Control low-level client, we need to make it possible to pass a CancellationToken to any HLC method by calling the LLC version.

LLC Considerations
Per @KrzysztofCwalina, we would like the models to be designed such that even if they are in the main package, they can be used with the protocol methods, i.e. given the following protocol method and helper method:

public virtual Response Foo(RequestContent content, RequestOptions options);
public virtual OutputModel Foo(InputModel model);

We should allow:

var input = new InputModel();
OutputModel output = client.Foo(input, new RequestOptions());

To make this work for the input is easy. We just need a cast from InputModel to RequestContent.

Similarly, we could add a cast from Response to OutputModel to make the output work, but in addition it would be great if the error behavior was exactly the same whether the protocol method or the grow up method was called. This means the cast from Response to OutputModel would have to have access to ClientDiagnostics.

Implementation
To accomplish this, we’ll add the following to Core, as specified by @pakrym:

Add Response.Throw() method that produces the same exception as the full data-plane client would for the same response.

So these two examples produce the same exception when the service returns 404:

var response = llc.GetSomething();
response.Throw();

dataPlane.GetSomething();

The call to response.Throw() can replace any line throw _clientDiagnostics.CreateRequestFailedException(message.Response); in generated/HLC code.

[annelo-msft]

Per recent discussion, we'll do this as:

• Adding a property to Response to indicate if an exception should be thrown (for now, I’m going to call this IsError)
• Adding a constructor to RequestFailedException that reads internal properties of Response to format the exception in default or custom ways
• Using the “default classifier” – i.e. the one generated from the REST API spec – to inform the semantics of Response.IsError

Relevant PR making API additions to ResponseClassifier: https://github.com/Azure/azure-sdk-for-net/pull/22923/files

[annelo-msft]

We will not resolve API questions as part of this change.

[annelo-msft]

Relevant Draft PR investigations:

• LLC investigation into casting Response type to models #23339
• Intial set-up for llc tests #23909

[annelo-msft]

Depends on: #24051

[annelo-msft]

Design issues currently being discussed here: #24164

[annelo-msft]

Relevant PRs have merged.