MM-53924 - Implement NotificationWillBePushed plugin hook (#24263)

* add NotificationWillBePushed hook

* mocks

* use a struct for hook parameters; simplify number of parameters sent across RPC

* missing wg.Wait

* change to a bool return value

server/channels/app/test_templates/hook_notification_will_be_pushed.tmpl

@@ -0,0 +1,18 @@
+package main
+
+import (
+    "github.com/mattermost/mattermost/server/public/plugin"
+    "github.com/mattermost/mattermost/server/public/model"
+)
+
+type MyPlugin struct {
+    plugin.MattermostPlugin
+}
+
+func (p *MyPlugin) NotificationWillBePushed(notification *model.PluginPushNotification) (cancel bool) {
+    %s
+}
+
+func main() {
+    plugin.ClientMain(&MyPlugin{})
+}