feat!: Make the url signing secret a mandatory config option

This is required for allowing the web office to download images to
insert into documents.

The secret is generated by `opencloud init` and the server refuses
to start now without a secret being set. (Breaking Change)

Also the setting is now moved to the shared options as all involved
services need the same secret to work properly.

Related: https://github.com/opencloud-eu/web/issues/704

services/proxy/pkg/config/parser/parse.go

@@ -56,9 +56,14 @@ func Validate(cfg *config.Config) error {
 	if cfg.ServiceAccount.ServiceAccountID == "" {
 		return shared.MissingServiceAccountID(cfg.Service.Name)
 	}
+
 	if cfg.ServiceAccount.ServiceAccountSecret == "" {
 		return shared.MissingServiceAccountSecret(cfg.Service.Name)
 	}
 
+	if cfg.Commons.URLSigningSecret == "" {
+		return shared.MissingURLSigningSecret(cfg.Service.Name)
+	}
+
 	return nil
 }