MM-53924 - Implement push notifications plugin hook and plugin api method (#24350)

* Revert "MM-52804 - Implement SendPushNotification plugin api method (#24273)"

This reverts commit 8418eefb75.

* Revert "MM-53924 - Implement NotificationWillBePushed plugin hook (#24263)"

This reverts commit f13a531bca.

* implement NotificationWillBePushed plugin hook

* implement SendPushNotification plugin api method

* move where we're setting post and channel type

* fix comment

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/test_templates/hook_notification_will_be_pushed.tmpl

@@ -1,15 +1,15 @@
 package main
 
 import (
-    "github.com/mattermost/mattermost/server/public/plugin"
-    "github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/plugin"
 )
 
 type MyPlugin struct {
     plugin.MattermostPlugin
 }
 
-func (p *MyPlugin) NotificationWillBePushed(notification *model.PluginPushNotification) (cancel bool) {
+func (p *MyPlugin) NotificationWillBePushed(notification *model.PushNotification, userID string) (*model.PushNotification, string) {
     %s
 }