Cosmos: Upsert

This is the example SDK code for upsert:

```C#
Product item = new(
id: "68719518391",
category: "gear-surf-surfboards",
name: "Yamba Surfboard",
quantity: 12,
price: 850.00m,
clearance: false
);

ItemResponse response = await container.UpsertItemAsync(
item: item,
partitionKey: new PartitionKey("gear-surf-surfboards")
);
```

Obviously, related to #4526, and we should align experience and API, but I expect the implementation will be quite Cosmos-specific, hence a separate issue.