Allow user to customize update+update merge

[frederikaalund]

The existing mutation merging works really well already. I'd like to build on top of it.

Specifically, I'd like to have an update + update that results in removed. Consider this sequence of events:

1. There is a row with name = "fred"
2. End user changes it so name = "ben"
3. End user changes it again so name = "fred"

Overall, the changes cancel each other out. In other words: update + update results in no change at all.

I don't expect TanStack DB to deal with this case in general. Instead, I propose an addition to the API that allows the user to customize the update + update case. This way, it's up to the user to implement it properly.

Any thoughts on this?