Refine Azure plugin to generate more accurate SDK for Azure

[live1206]

Putting together remaining issues/comments from initial PR

• In CreateXXXRequest method implementation, RequestContext should be used for HttpMessage creation
• Classifier2xxAnd4xx needs further refinement
• Authentication policy is missing for HttpPipeline
• Avoid RequestMethod instance allocation
• request header should be set instead of add
• Get rid of ClientUriBuild for Azure, set the Uri in the request directly
• CancellationToken should be passed from RequestContext to pipeline.Send
• Sync on the pattern of ExtractResponseContent
• Remove ErrorResult from SCM