replica.origin cannot use the Barman Cloud Plugin — only legacy barmanObjectStore or pg_basebackup are supported

cluster.externalClusters (in templates/_helpers.tpl / the template that builds originCluster for mode: replica) only generates two possible shapes for the replica's origin external cluster:

```
{{- if not (empty .Values.replica.origin.objectStore.provider) }}
barmanObjectStore: ...
{{- end }}
{{- if not (empty .Values.replica.origin.pg_basebackup.host) }}
{{- include "cluster.externalSourceCluster" .Values.replica.origin.pg_basebackup | nindent 4 }}
{{- end }}
```

Unlike the mode: recovery branch just above it — which has an explicit plugin case (pluginRecoveryCluster, gated on recovery.method == "plugin") — there is no equivalent branch here for a plugin-based origin. This means replica.origin has no way to render:

```
externalClusters:
- name: originCluster
plugin:
name: barman-cloud.cloudnative-pg.io
parameters:
barmanObjectName:
serverName:
```

Start in templates/_helpers.tpl at the cluster.externalClusters logic that builds originCluster for mode: replica, then compare it with the explicit plugin branch in the mode: recovery section. Add support for the plugin-based replica.origin shape, and verify that rendering produces the shown plugin external cluster with the configured object and source cluster names.

Written by the indexing model from the issue text.