Expose type for naming servers when adding additional network listeners

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.

Start at the Microcluster.AddServers API and inspect how additional listener names and the reserved core and unix names are represented. The work is complete when downstream callers have an exposed server-name type that makes the naming constraint clear and the relevant Go tests or API checks pass.

Written by the indexing model from the issue text.