Move plugins under Channels (#18938)

* Move plugins under Channels

We move all plugin related fields under *Channels.
This essentially migrates several methods from being under
*Server to under *Channels.

We also move the plugin startup and shutdown code
to be under Channels.Start and Channels.Shutdown.

While here, we remove the getPluginPublicKeyFiles
method which was a one-line method which uselessly
returned an error.

Lastly, we fix the product initialization order which
was incorrect previously. Products are dependent on
the main server.

So startup should be products -> server.
And shutdown should be server -> products.

```release-note
NONE
```

* Added app layer

```release-note
NONE
```

* Incorporate suggestions

```release-note
NONE
```

app/plugin_event.go

@@ -9,10 +9,10 @@ import (
 	"github.com/mattermost/mattermost-server/v6/model"
 )
 
-func (s *Server) notifyClusterPluginEvent(event model.ClusterEvent, data model.PluginEventData) {
+func (ch *Channels) notifyClusterPluginEvent(event model.ClusterEvent, data model.PluginEventData) {
 	buf, _ := json.Marshal(data)
-	if s.Cluster != nil {
-		s.Cluster.SendClusterMessage(&model.ClusterMessage{
+	if ch.srv.Cluster != nil {
+		ch.srv.Cluster.SendClusterMessage(&model.ClusterMessage{
 			Event:            event,
 			SendType:         model.ClusterSendReliable,
 			WaitForAllToSend: true,