This is a port of Spring MVC CORS support for Spring Web Reactive:
 - CORS classes keep the same name but are in the
   web.cors.reactive package
 - CorsConfiguration is reused because not tied to Servlet API
 - CORS HandlerMapping integration is done at
   AbstractHandlerMapping level
 - AbstractUrlHandlerMapping and AbstractHandlerMethodMapping
   have been slightly modified to call
   AbstractHandlerMapping#processCorsRequest()
 - Both global CORS configuration + @CrossOrigin support have
   been implemented

Issue: SPR-14545