update handler to accept optional NextResponse for nextjs 13 app rout…

[pengyanb]

• Minor update to the handler signature to accept an optional NextResponse parameter.
• Minor update to README.md file, to demonstrate how cookies can be added to the res object in the "graphql resolver".

[trevor-scheer]

@martinnabhan would you mind taking a look at this one?

[martinnabhan]

I'm not sure how I feel about passing a NextResponse to the context just for the sake of copying its headers (and cookies) onto the actual response? Might cause confusion when someone tries to use res for anything else, since res won't be the actual response object.

Instead I suggest we might add a headers property to the default context, which will have its contents copied onto the final response.

What does everyone think?

[luchillo17]

This PR has been stuck for over 6 months, and is outdated against the latest version and all, @pengyanb do you think you can check the latest version and see if this PR is still valid?