Return mutationKey from useMutation

[Jak-Ch-ll]

This is a bit of a follow-up to #2304.

Let's consider an example like this:

const mutationKey = ['foo']

function useUpload() {
  return useMutation({
    mutationKey,
    ...
  })
}

function useIsUploading() {
  return useIsMutating({ mutationKey })
}

Right now, if I don't want to hard code the mutationKey twice I have to save it outside of the composable so it can be shared.

I would like the mutation composable to be the single source of truth and expose the mutationKey for useIsMutating and useMutationState to reuse. It could be used like this:

function useUpload() {
  return useMutation({
    mutationKey: ['foo'],
  })
}

function useIsUploading() {
  const { mutationKey } = useUpload()
  return useIsMutating({ mutationKey })
}

This would be especially useful when the mutationKey has dynamic parts.

While writing this I even thought of a possible extension of this concept in the future:

function useUpload() {
  return useMutation({
    mutationKey: ['foo'],
  })
}

function useUploadingState() {
  return useMutationState(useUpload)
}

This would allow the useMutationState to inherit its typings from the passed useMutation.

[TkDodo]

const uploadOptions = mutationOptions({
  mutationKey: ['foo'],
  mutationFn: () => ...
})

useMutation(uploadOptions)
useIsMutating(uploadOptions)