Support optimisticData as a function

[shuding]

Discussed in #1850

^{Originally posted by ludovic-montel February 9, 2022}
First of all, thanks a lot for this library. I've been using it for almost a year, and more intensively for several months, and I love it!

Il like to create optimistic user interfaces, and the new optimisticData option with its auto-rollback behavior is great!

I'd like to push it a little further: could you optimisticData as a function ? I think it would be consistent with the mutate first argument and with #1817.

This function would take the cached data as parameter, and would allow:

• using the global mutate function
• extracting and test a pure function that makes the mutation I want, depending on the current data and the operation that is processed

Again, thanks a lot for your work