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>

values.yaml

@@ -253,9 +253,17 @@ initPreScript: ""
 #
 ## @param signing.enabled Enable commit/action signing
 ## @param signing.gpgHome GPG home directory
+## @param signing.privateKey Inline private gpg key for signed Gitea actions
+## @param signing.existingSecret Use an existing secret to store the value of `signing.privateKey`
 signing:
   enabled: false
   gpgHome: /data/git/.gnupg
+  privateKey: ""
+  # privateKey: |-
+  #   -----BEGIN PGP PRIVATE KEY BLOCK-----
+  #   ...
+  #   -----END PGP PRIVATE KEY BLOCK-----
+  existingSecret: ""
 
 ## @section Gitea
 #