This ticket is to introduce the registry for replica set ddl observers/hooks and create it/register it on the service context during mongod startup. We should also introduce the interface for the hooks/observers at the same time. The registry object should provide an ability to register observers/hooks, an onBeginDDL and onEndDDL hook which calls this function on any of the registered observers/hooks, and the ability to get specific hooks/observers out by name. It additionally should provide a scoped object which can be used by DDLS to guard their execution with the constructor calling onBegin and the destructor calling onEnd.