Add get_available_sorts() to the Drupal_Solr_Query_Interface interface

Over on #440788-3: Patch for 3 small errors - $id, get_filters(), and apachesolr_modify_query/OG problem I identifying a problem with the apachesolr_block() function. It calls $query->get_available_sorts() but that function isn't part of the Drupal_Solr_Query_Interface interface. This causes problems for the apachesolr_views module because the apachesolr_views_query class extends views_plugin_query and can't extend Solr_Base_Query. It seems to me that apachesolr_block() should either not call a function that's not part of the interface or it should be added to the interface.