These changes enable tracing of Pulumi API calls.
The span with which to associate an API call is passed via a
`context.Context` parameter. This required plumbing a
`context.Context` parameter through a rather large number of APIs,
especially in the backend.
In general, all API calls are associated with a new root span that
exists for essentially the entire lifetime of an invocation of the
Pulumi CLI. There were a few places where the plumbing got a bit hairier
than I was willing to address with these changes; I've used
`context.Background()` in these instances. API calls that receive this
context will create new root spans, but will still be traced.
This command cancels a stack's currently running update, if any. It can
be used to recover from the scenario in which an update is aborted
without marking the running update as complete. Once an update has been
cancelled, it is likely that the affected stack will need to be repaired
via an pair of export/import commands before future updates can succeed.
This is part of #1077.
