Add Implicit CorsConfigurationSource/PreFlightRequestHandler Detection to <cors> XML Namespace

[jzheaux]

CorsConfigurer (the Java/lambda HttpSecurity.cors() DSL) auto-detects a
CorsConfigurationSource (via the mvcHandlerMappingIntrospector bean) or a
PreFlightRequestHandler bean when neither is configured explicitly (added
in #18926). The XML <cors> namespace element has no equivalent: without a
configuration-source-ref or ref attribute, CorsBeanDefinitionParser
throws a BeanCreationException.

Add a CorsFilterFactoryBean, used by CorsBeanDefinitionParser when
configuration-source-ref is omitted, that mirrors CorsConfigurer's
detection logic — so XML configuration has the same implicit-bean-detection
behavior as Java configuration.

Related to #18926.