MM-21103: change plugin signature path (#13360)

* MM-21103: change plugin signature path

Save as `<plugin_id>.tar.gz.sig` instead of `<plugin_id>.sig`. The latter was a relic of the previous design to support multiple plugin signatures, but now creates an inconsistency with how the original source files were supplied as `<some_name>.tar.gz` and `<some_name>.tar.gz.sig`.

Fixes: https://mattermost.atlassian.net/browse/MM-21103

* relax signature matches to avoid assuming signatures always exist

app/plugin_install.go

@@ -405,5 +405,5 @@ func (a *App) getBundleStorePath(id string) string {
 }
 
 func (a *App) getSignatureStorePath(id string) string {
-	return filepath.Join(fileStorePluginFolder, fmt.Sprintf("%s.sig", id))
+	return filepath.Join(fileStorePluginFolder, fmt.Sprintf("%s.tar.gz.sig", id))
 }