unstructured client should key off of the unstructured interface

[DirectXMan12]

If we key off of the unstructured interface instead of the type, we can allow for alternate internal unstructured representations, and the underlying machinery works fine with the interface.

[fejta-bot]

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/lifecycle stale

[shawn-hurley]

/remove-lifecycle stale
/lifecycle frozen

[vincepri]

/priority awaiting-more-evidence
/help

We might need a spike to understand what we learned from #468.

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In response to this:

/priority awaiting-more-evidence
/help

We might need a spike to understand what we learned from #468.

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