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_test.go

@@ -537,7 +537,7 @@ func TestPluginSync(t *testing.T) {
 			pluginFileReader, err := os.Open(filepath.Join(path, "testplugin.tar.gz"))
 			require.NoError(t, err)
 			defer pluginFileReader.Close()
-			_, appErr = th.App.WriteFile(pluginFileReader, th.App.getBundleStorePath("testplugin.tar.gz"))
+			_, appErr = th.App.WriteFile(pluginFileReader, th.App.getBundleStorePath("testplugin"))
 			checkNoError(t, appErr)
 			// no signature
 			appErr = th.App.SyncPlugins()