Adds `pulumi update` so you can deploy to the Pulumi Console (via PPC on the backend).
As per an earlier discussion (now lost because I rebased/squashed the commits), we want to be more deliberate about how to bifurcate "local" and "cloud" versions of every Pulumi command.
We can block this PR until we do the refactoring to have `pulumi` commands go through a generic "PulumiCloud" interface. But it would be nice to commit this so I can do more refining of the `pulumi` -> Console -> PPC workflow.
Another known area that will need to be revisited is how we render the PPC events on the CLI. Update events from the PPC are generated in a different format than the `engine.Event`, and we'll probably want to change the PPC to emit messages in the same format. (e.g. how we handle coloring, etc.)
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