Propagate trace context to webhook and upload requests
If the request to /predict contains headers `traceparent` and
`tracestate` defined by w3c Trace Context[1] then these headers are
forwarded on to the webhook and upload calls.
This allows observability systems to link requests passing through cog.
[1]: https://www.w3.org/TR/trace-context/
Signed-off-by: Aron Carroll <aron@replicate.com>
