Allow set of userDetailsChecker

It is not possible to customize the `userDetailsChecker` in [CasAuthenticationProvider](https://github.com/spring-projects/spring-security/blob/bdc3feaa6d4e63eb7b696418c38848177ad37337/cas/src/main/java/org/springframework/security/cas/authentication/CasAuthenticationProvider.java#L69). 

Would be possible to add a setter method (both in 5.6.x and 5.5.x) ? 

