PendingFragmentContent
using fastly::esi::PendingFragmentContent = typedef std::variant<http::request::PendingRequest, http::Response, std::monostate>Alias for std::variant<http::request::PendingRequest, http::Response, std::monostate>.
Content that can be returned from a fragment request dispatcher. This can either be a pending request, a response, or an empty value to indicate that no content is available.
