Generate types for loadingState exposed for deferred queries

[clarencenpy]

With @defer support coming to Apollo Client, a new field loadingState is exposed on ApolloCurrentResult, so that users are able to use it to distinguish between null and pending states for any field. Reference: apollographql/apollo-client#3686

loadingState mirrors the shape of the data that is returned from the query. For example, for a query below:

query {
  newsFeed {
    stories {
      title
      comments @defer {
        text
      }
    }
  }
}

The data returned as part of the initial response looks like this:

data: {
  newsFeed: {
    stories: {
      title: "...",
      comments: null,
    }
  }
}

And the loadingState looks like this:

loadingState: {
  newsFeed: {
    stories: {
      title: true,
      comments: undefined,
    }
  }
}

After all patches resolve, the leaf nodes of loadingState would all be set to true.

Therefore, the type associated with loadingState has the same structure as data, except that its leaf nodes are typed as undefined | boolean rather than GraphQLOutputType. It will be really nice to have codegen support for this so that users can get autocomplete on the loadingState too.