These changes add the ability to export a stack's latest deployment or
import a new deployment to a stack via the Pulumi CLI. These
capabilities are exposed by two new verbs under `stack`:
- export, which writes the current stack's latest deployment to stdout
- import, which reads a new deployment from stdin and applies it to the
current stack.
In the local case, this simply involves reading/writing the stack's
latest checkpoint file. In the cloud case, this involves hitting two new
endpoints on the service to perform the export or import.
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