Report whether a request includes a HTTP_COOKIE object with a specified
cookie_name, and optionally that the cookie passes
.req_has_cookie_impl(request, cookie_name, validation_fn, ...)A length-1 logical vector.
A shiny request object.
The cookie that must be present, as a length-1 character vector.
A function that takes the value of the cookie as the
first parameter, and returns TRUE if the cookie is valid, and FALSE
otherwise.
Additional parameters passed on to validation_fn.