Expose type for naming servers when adding additional network listeners

[mas-who]

When adding additional network listeners in a downstream project using (microcluster.Microcluster).AddServers, we need to pass in a map of the type map[string]rest.Server. Since there are reserved names for microcluster internal network listeners such as core and unix, we should provide a server name type so that it is clear to the downstream project server names should be chosen with care.