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_requests.go

@@ -21,7 +21,7 @@ import (
 )
 
 func (s *Server) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
-	pluginsEnvironment := s.GetPluginsEnvironment()
+	pluginsEnvironment := s.Channels().GetPluginsEnvironment()
 	if pluginsEnvironment == nil {
 		err := model.NewAppError("ServePluginRequest", "app.plugin.disabled.app_error", nil, "Enable plugins to serve plugin requests", http.StatusNotImplemented)
 		s.Log.Error(err.Error())
@@ -46,7 +46,7 @@ func (s *Server) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
 }
 
 func (a *App) ServeInterPluginRequest(w http.ResponseWriter, r *http.Request, sourcePluginId, destinationPluginId string) {
-	pluginsEnvironment := a.GetPluginsEnvironment()
+	pluginsEnvironment := a.ch.GetPluginsEnvironment()
 	if pluginsEnvironment == nil {
 		err := model.NewAppError("ServeInterPluginRequest", "app.plugin.disabled.app_error", nil, "Plugin environment not found.", http.StatusNotImplemented)
 		a.Log().Error(err.Error())
@@ -90,7 +90,7 @@ func (s *Server) ServePluginPublicRequest(w http.ResponseWriter, r *http.Request
 	vars := mux.Vars(r)
 	pluginID := vars["plugin_id"]
 
-	pluginsEnv := s.GetPluginsEnvironment()
+	pluginsEnv := s.Channels().GetPluginsEnvironment()
 
 	// Check if someone has nullified the pluginsEnv in the meantime
 	if pluginsEnv == nil {