Provide onMutate on MutationCache

[stefanprobst]

hi, what do you think about a global onMutate cb on the MutationCache (in addition to onSuccess and onError)? i think it could be helpful for e.g. managing toast notifications for mutations in one place.

[TkDodo]

hm, is it really a use-case to show a toast notification when a mutation starts? I mean maybe, since react-hot-toasts allows for loading spinners inside the toast 🤔

[stefanprobst]

yes i was thinking about react-hot-toasts's toast.promise

[TkDodo]

I think I'd accept a PR in this direction :)