Add simpler Exec methods

[qmfrederik]

The Kubernetes API currently has a WebSocketNamespacedPodExecAsync method, which gets you a WebSocket, but doesn't give you any clues as how to use it.

This PR adds a new NamespacedPodExecAsync method which:

• Lets you pass a callback which can process the stdin/stdout/stderr streams
• Supports the Kubernetes error stream, from which you can read the process exit code

To simplify testing, I also extracted an IStreamDemuxer interface which helps you mock the StreamDemuxer class.

[brendandburns]

@qmfrederik can you try rebasing this PR to re-trigger CI?

Thanks!

[tg123]

@brendanburns @qmfrederik there is a button to rerun the build again.
however this failure is due to resharp ran more than 1 hour
anyway to apply more credit from azure team?

[brendandburns]

/lgtm
/approve

I'll looking into getting us some build credits.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brendandburns, qmfrederik

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