populate a new 'serverRevision' field for the version response

[devoncarew]

• populate a new serverRevision field for the version response

This can help us know which revision a server is serving. I believe that the cloud K_REVISION env var - when deployed via a github trigger - is the git SHA value.

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[devoncarew]

Hmm, it looks like it's being populated with dart-service-cloud-run-00638-c9f; I may follow up this PR w/ one that plumbs through the git SHA of the version that was deployed.

[parlough]

Sounds good! A commit hash does sound like it'd be easier and more useful. It seems Cloud Build has a substitution of the trigger commit hash you could pipe through if we don't already have access to it somewhere else: https://cloud.google.com/build/docs/configuring-builds/substitute-variable-values