Cosmos Store

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Replaces #20, which hosted a former version of this work (with more legible history wrt early development phases).

Given anything that reflected a cross-cutting need was already merged early and the rebased upon, the earlier commits don't compile cleanly (hence doing a squash merge).

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That etag retrieval trickery was a lifesaver - thanks @jinglegit !
(I had been assigning to a temp with scope outside the callback, which drops the value by the time I tried to put it in the response)
Looking great now

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@dongdongcai @enricosada @eulerfx @jinglegit @michaelliao5 @troykershaw Hold on to your posteriors, pipelines are smooth, this thing is going to master 👏