Feature Request: Automatically configure "concurrent sessions" support

See https://docs.spring.io/spring-security/site/docs/current/reference/html5/#concurrent-sessions

I've implemented this in our Grails application using a combination of `resources.groovy` and `BootStrap.groovy` using `clientRegisterFilter`, but it seems like it would fit right in with this plugin as a configuration option.

I'm thinking something like `grails.plugin.springsecurity.trackConcurrentSessions: true|false`. It can be false by default.

I'm happy to contribute this feature as a PR if you think it's worthwhile.

Start with the Spring Security concurrent-sessions documentation linked in the issue, then inspect the resources.groovy and BootStrap.groovy integration using clientRegisterFilter. The feature should expose the proposed grails.plugin.springsecurity.trackConcurrentSessions option, default it to false, and enable the support when set to true without manual setup.

Written by the indexing model from the issue text.