ClientModel: Add transport and buffering policy to System.ClientModel

[annelo-msft]

Add the implementation of the HttpClient transport to the System.ClientModel library, as well as the ResponseBufferingPolicy. Together these provide a functional default pipeline (without retries) so we can add functional tests of the pipeline as well.

Please note that we're working on an outstanding investigation regarding whether we can move the buffering and network timeout logic out of the response buffering policy and into the transport. That work is tracked by #41198. Depending on the outcome of that investigation, we may follow up and remove the ResponseBufferingPolicy entirely. The functional implementation would remain the same just in a different place, so it is still worth reviewing in this PR in order to make forward progress on moving ClientModel into main prior to this investigation being complete.

This PR adds the public types:

• HttpClientPipelineTransport
• ResponseBufferingPolicy (pending removal based on the outcome of ClientModel: Investigate feasibility of moving response buffering into the transport #41198)

and internal types:

• PipelineRequestHeaders
• PipelineResponseHeaders
• HttpClientPipelineTransport.HttpPipelineRequest
• HttpClientPipelineTransport.HttpPipelineResponse

[azure-sdk]

API change check

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System.ClientModel