feat(Cosmos): Handle blind write of events+unfolds idempotently

[bartelink]

Stored proc extension to remove redundant reads when writers such as CosmosStoreSink are syncing unfolds and events

Leaving as a draft for now for the following reasons:

• not a massive perf win
• existing API returns the conflicting events; ideally the writer should verify that the events are equivalent (if someone was using the EventsContext API and not working based on cached etags, then you'd want something equivalent to the EventStoreDB EventId checks)
• the more complete answer is to tag the unfolds with a revision value, and have the idempotency be based on that matching and/or unfolds being completely empty (as it stands we don't have a good way to nullify the write costs when retraversing)