This PR is a proof of concept of adding lazy evaluation support to
graphql-core.

It follows a similar API to [graphql-ruby](https://graphql-ruby.org/schema/lazy_execution.html)
and it allows the developer to define "lazy" type to enable batching
without using asyncio. The tests illustrate how this can be used to enable
a dataloader pattern.

N.B. The DeferredValue object is very similar in functionality to the
Promise library from graphql-core v2. I decided to reimplement a
subset rather than use it directly though because it's scope is bigger
than what I needed. It's a purely internal implementation detail though
and can be replaced in future.