(v4) Be able to overwrite variables in mutate

[kawamataryo]

Thank you for a very useful library.

I couldn't overwrite the variable specified by the initial value in mutate, so I fixed it.

When I wrote the following code, I couldn't overwrite a variable.
I think it would be better to make it overwritable, how about that?

   const DELETE_PRODUCT = gql`
    mutation DeleteProduct($id: Int!) {
        delete_products(where: {id: {
            _eq: $id
        }}){
            returning {
                id
                name
            }
        }
    }
   `
    const { mutate: deleteProduct } = useMutation<
      DeleteProductMutation,
      DeleteProductMutationVariables
    >(DELETE_PRODUCT, () => ({
      variables: {
        id: 0
      },
      refetchQueries: [{ query: FETCH_PRODUCTS }]
    }));

    const onDeleteProduct = (productId: number) => {
      deleteProduct({ id: productId }); 
      // At the time of request, the id of the variable will be the '0' specified at the time of useMutation instead of the value of this productId.
    };

I could do the following, but I'd have to use any to avoid type errors.

    const { mutate: deleteProduct } = useMutation<
      DeleteProductMutation
    >(DELETE_PRODUCT, () => ({
      refetchQueries: [{ query: FETCH_PRODUCTS }]
    }));

    const onDeleteProduct = (productId: number) => {
      deleteProduct({ id: productId } as any); 
    };

[bbugh]

This looks great to me, I can see how that would be a problem. 👍

[kawamataryo]

@Akryum

Could you please review it.
If you have any problems with merging, let me know and I'll fix it.