refactor(ci): Introduce common, re-usable quickstart workflow

[ncooke3]

It's not a red diff, but is cleaner. I've added more functionality like automatic uploads of failed quickstart folders and we have a central Xcode configuration hook for quickstart tests.

#no-changelog

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[ncooke3]

checkpoint: abtesting works.

[ncooke3]

checkpoint: abtesting, rtdb, auth

[daymxn]

+1, but I defer to other folks for final approval -- as I still lack sufficient context on infra material.