simple, functional interface to add additional balancer lb selection methods

without requiring code changes to mod_proxy/mod_proxy_balancer;
these can be implemented via sub-modules now.

Let the games begin...


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@232282 13f79535-47bb-0310-9956-ffa450edef68