add Upsert functions

[mzy666888]

Problem / motivation

Can you add a method: Upsert, to automatically select insertion or update based on the state of the model.

When there is a list, some data may have changed or new data may have been added. At this point, it is possible to autonomously decide whether to insert or update by calling Upsert.

thank you!

Proposed solution

I'm not sure.

Alternatives considered

No response

Additional context

No response

[mrdevrobot]

Thanks for the suggestion! Added `UpsertAsync`/`UpsertBulkAsync` to both the typed (`DocumentCollection<TId,T>`) and schema-less (`DynamicCollection`) APIs — see #142.

It's not a `FindByIdAsync` + `InsertAsync`/`UpdateAsync` wrapper: each document is resolved with a single primary-index lookup inside one transaction (insert if the id is unset/absent, replace if found), returning `UpsertResult` so you know which happened.

Will be available starting from v5.1.0.