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

api4/plugin_test.go

@@ -942,7 +942,7 @@ func TestInstallMarketplacePlugin(t *testing.T) {
 		require.Equal(t, "testplugin_v2", manifest.Id)
 		require.Equal(t, "1.2.3", manifest.Version)
 
-		filePath := filepath.Join(*th.App.Config().PluginSettings.Directory, "testplugin_v2.sig")
+		filePath := filepath.Join(*th.App.Config().PluginSettings.Directory, "testplugin_v2.tar.gz.sig")
 		savedSigFile, err := th.App.ReadFile(filePath)
 		require.Nil(t, err)
 		require.EqualValues(t, sigFile, savedSigFile)