Update HandlerMappingIntrospector Usage in CORS support

[jzheaux]

The CorsConfigurationConfigurer should look up a PreFlightRequestHandler instead of a HandlerMappingIntrospector. It will still be registered under the same bean name in Framework 7, so that should remain as-is.

If a PreFlightRequestHandler is found by the given name, then safely cast it to CorsConfigurationSource and construct the CorsFilter as normal.

[jzheaux]

@evgeniycheban, this ticket is yours. Once you comment, I'll be able to assign it to you officially.

[evgeniycheban]

Hi, @jzheaux, please assign it to me.

[evgeniycheban]

Hi, @jzheaux I opened a PR to your fork.