FE list: optimistic updates

[TimBroddin]

❗Needs this merged first: #89902

Implement optimistic updates for batch methods inside client/data/plugins/use-update-schedules-mutation.ts

There's two places where we can update the queryClient cache:

• onMutate: here we can set the data to what we expect it to be
• onSettled: here we have the return data from each call as an array containing { siteSlug, response, error }. We can remove the errored entries here. Afterwards we invalidate multisite-schedules-update, which should then update to the same value we just set (if we nail it).

[TimBroddin]

Maybe the onMutate one isn't needed, as we don't redirect to the list until creation.