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 <sk.bunsenbrenner@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/374
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>

Makefile

@@ -6,3 +6,7 @@ prepare-environment:
 readme: prepare-environment
 	npm run readme:parameters
 	npm run readme:lint
+
+.PHONY: unittests
+unittests:
+	helm unittest --helm3 --strict -f 'unittests/**/*.yaml' ./