Add a simple "merge" function option to control how local and shared states are synced

[cyrilboyer]

Hi, I’m proposing a merge option (function) to give more control over how local and shared states are combined on updates.

I initially made it to fix my use case with feathers-pinia where services states get overridden by ones with less data due to how pagination and data is stored nested (when going from a list page to a single item's page for example).

I tried to add it in a similar way as the existing serializer option, it allows to write custom logic for merging states, with access to store IDs and keys for exceptions.

Let me know if that makes sense, thanks!