Refactor Kerberos UserDetailsService to shared utility module

Some minor classes are duplicated (KerberosUserDetailsService and AlternateKerberosUserDetailsService) because they are needed in different modules (nifi-kerberos-iaa-provider and nifi-web-security) that cannot have dependencies on each other. These classes are only 6 lines of code each, but should be refactored to a shared utility module.