Allow using network=external option in cbbackupmgr, cbexport, cbimport

This request is coming from CAO, k8s users who want to use cbbackupmgr, cbimport, cbexport from an external network (outside the Kubernetes cluster).  Currently, you need to specify the exact alternate address of a Couchbase node or using alternate addresses with cbbackupmgr, cbimport, cbexport won't work, but since the pod's alternate address changes frequently (as pods are designed to be created, destroyed), it's impractical to use cbbackupmgr, cbimport, cbexport from an external network.  The ask is to allow using an option like the SDK's, where network=external can be used with cbbackupmgr, cbimport, cbexport, to indicate that alternate addresses should be used for the operation.   The --cluster option to cbbackupmgr/cbimport/cbexport would be given the service endpoint which will allow the initial connection to the cluster for the bootstrapping info to get the current alternate addresses.

For additional info, see the discussion in CBSE-10413 and specific comments like this one –  https://issues.couchbase.com/browse/CBSE-10413?focusedCommentId=525963&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-525963