Re-add GPG configuration feature (#374)

This reverts d5ce1a47ea and therefore adds the GPG feature back into main.
As it is a breaking change, this PR now also contains the required upgrade notes.

Closes #107 again.

Co-authored-by: justusbunsi <[email protected]>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/374
Reviewed-by: pat-s <[email protected]>
Reviewed-by: John Olheiser <[email protected]>

templates/_helpers.tpl

@@ -331,3 +331,7 @@ https
     {{- toYaml .Values.extraVolumeMounts -}}
   {{- end -}}
 {{- end -}}
+
+{{- define "gitea.gpg-key-secret-name" -}}
+{{ default (printf "%s-gpg-key" (include "gitea.fullname" .)) .Values.signing.existingSecret }}
+{{- end -}}