[JS] utils package is missing response types

For example, there is a type for `HTTPRequest` but not one for `HTTPResponse`. It's easy enough to type yourself, but it was hard to find in the [docs](https://docs.aws.amazon.com/appsync/latest/devguide/tutorial-http-resolvers.html) what the actual structure of the response object is.

Start in the utils package by locating the existing HTTPRequest type, then compare it with the HTTP resolver response structure in the linked AWS AppSync documentation. Add the corresponding HTTPResponse type and verify that consumers can use it instead of declaring the response shape themselves.

Written by the indexing model from the issue text.