Add ability to pass in custom parent span to SDK operation

It would be good if users were able to pass in their own custom parent spans into SDK operations.

The use-case would be where a single application API call could perform multiple Couchbase operations and you want to track both the individual operation latencies, but also the overall API call latency.
This would make it easier to correlate specific sets of Couchbase operations that were slow which result in performance degradation across the whole application operations.