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_shutdown_test.go

@@ -63,7 +63,7 @@ func TestPluginShutdownTest(t *testing.T) {
 	done := make(chan bool)
 	go func() {
 		defer close(done)
-		th.App.Srv().ShutDownPlugins()
+		th.App.ch.ShutDownPlugins()
 	}()
 
 	select {