Provide KV type in the interop/storage package

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I see this:

	newNodes := storage.Find(ctx, newNodesPrefix, storage.None)
	for iterator.Next(newNodes) {
		newNode := iterator.Value(newNodes).(struct {
			key []byte
			val []byte
		})

Especially given the fact that this is not the first time I'm seeing it. We have similar code and types in other contracts dealing with updates or otherwise interacting with iterators returning KV pairs.

Describe the solution you'd like

Provide some type for KV pair.

Describe alternatives you've considered

Keep as is, explain and duplicate it again and again.

Additional context

nspcc-dev/neofs-contract#438