Track external repo commit hashes for external configs

[jaredlockhart]

Now that we store external config files right in the Experimenter repo, it would be nice to know which commit in the external repo the current version was fetched from. So we should stick a comment at the top of each external config file (jetstream configs, feature manifests) that says what the latest commit hash of the external repo it was fetched from is at the time it was fetched. This will make it easier to reason about which version was used at some point in time.

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