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.
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