Add explicit namespace management to Helm chart

[guimou]

Summary

Follow the pattern established in rh-ai-community-plugins/brewet#206 to add explicit namespace management to the admin plugin's own Helm chart.

Changes needed

• Add a namespace helper in chart/templates/_helpers.tpl: .Values.namespace | default .Release.Namespace
• Add a namespace.yaml template so the chart creates the namespace itself
• Add namespace: cp-plugins-admin default in chart/values.yaml
• Explicitly set namespace: {{ include "community-plugins-admin.namespace" . }} on every namespaced resource template (deployments, services, service accounts, etc.)

Why

Ensures the chart is self-contained for namespace creation and consistent with other community plugins. The values.yaml default acts as documentation and a fallback for manual deployments, while the admin plugin's lifecycle service reads the namespace from plugin.yaml when deploying automatically.

Reference

• Brewet PR: Default namespace brewet#206
• Admin plugin plugin.yaml already declares namespace: cp-plugins-admin in the install section

[guimou]

Done in #111