ClientModel: Add unit tests to validate policy ordering in default pipeline construction logic for Azure.Core and ClientModel pipelines

[annelo-msft]

Evaluate whether Azure.Core HttpPipeline can use the same logic as ClientPipeline for creating the default pipeline – to ensure default pipelines have same policy orderings. Should HttpPipeline inherit from ClientPipeline?

@tg-msft FYI

[annelo-msft]

@KrzysztofCwalina has investigated this, and reported it is complicated and viral - because Policy types are different and don't inherit. We will solve this problem by adding unit tests to validate that the default pipelines are in sync.