add ConfigSetService extension ability

I want to load schema and solrconfig configuration form DB，that need to customize the SolrResourceLoader for my own。

customized SolrResourceLoader will be create from ConfigSetService.createCoreResourceLoader() method。

so i want to modify the static method ConfigSetService.createConfigSetService in that I can define the customized class name of ConfigSetService in solr.xml and create it in the method