REST Endpoint Example to allow cross-origin requests is not thread safe

HTTPBuilder is not thread safe by nature, which can create issues for clients who don't create a new HTTPBuilder instance on each request. At least one of our examples in the documentation uses a shared HTTPBuilder object that isn't recreated on each request. We should probably use the AsyncHTTPBuilder for this particular example, and create a new builder object within the method, rather than at the base of the script.