This takes the existing `apitype.Checkpoint` type and renames it to
`apitype.CheckpointV1` locking in the shape. In addition, we introduce
a `apitype.VersionedCheckpoint` type, which holds a version number and
a json document representing a checkpoint at that version. Now, when
reading a checkpoint, the CLI can determine if it's in a format it
understands, and fail gracefully if it is not.
While the CLI understands the older checkpoint version, it always
writes the newest version format, meaning that if you manage a
fire-and-forget stack with this version of the CLI, it will be
un-readable by previous versions.
Stacks managed by Pulumi.com are not impacted by this change.
Fixes: #887
936cab0c22