配置属性a: 1:不配置a matchifmissing=false 不满足 matchifmissing=true 满足
2:配置a=false matchifmissing=false 不满足 matchifmissing=true 不满足
3:配置a=true matchifmissing=false 满足 matchifmissing=true 满足

方式一、根据属性value，若value值为true，则加载此配置类 
@ConditionalOnProperty(prefix = "cas", value = "enable", matchIfMissing = true)
// 或者可以省略prefix前缀
@ConditionalOnProperty(value = "cas.enable", matchIfMissing = true)
方式二、根据属性name和havingValue指定值是否一致，一致则加载此配置类 
@ConditionalOnProperty(name = "cas.enable", havingValue = "true", matchIfMissing = true)