System.ClientModel: Provide APIs for message types to transfer dispose ownership of response

[annelo-msft]

Unblock generator work to generate protocol methods for streaming APIs where message.BufferResponse is set to false. This addresses a problem described by @AlexanderSher in https://gist.github.com/AlexanderSher/df22ed21acaa938a4371c188f0699b11.

To the end-user who calls a protocol method, what happens in the message's Dispose function is an implementation detail they're not aware of. We need to be able to hide this detail and allow the end-user to dispose the network stream by disposing Response -- i.e. we must be able to return an un-disposed Response to the end-user.

This PR adds an ExtractResponse method to PipelineMessage in ClientModel. When the Azure.Core/ClientModel integration occurs, this method will move to Azure.Core's HttpMessage as well, which will newslot it to return Response instead of PipelineResponse.

Protocol methods that implement streaming APIs must now:

1. Set message.BufferResponse = false
2. Call message.ExtractResponse prior to returning from the method

Convenience methods that call into these protocol methods should call ExtractResponse on the message if they are able to return the network stream to the end-user such that the end-user is able to dispose it. If the convenience method cannot pass dispose ownership to someone else (e.g. because it throws an exception or does not return the network stream because of the response status code), it should dispose the response itself.

[azure-sdk]

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Azure.Core
System.ClientModel