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/server.go

@@ -47,7 +47,6 @@ import (
 	"github.com/mattermost/mattermost-server/v6/einterfaces"
 	"github.com/mattermost/mattermost-server/v6/jobs"
 	"github.com/mattermost/mattermost-server/v6/model"
-	"github.com/mattermost/mattermost-server/v6/plugin"
 	"github.com/mattermost/mattermost-server/v6/services/awsmeter"
 	"github.com/mattermost/mattermost-server/v6/services/cache"
 	"github.com/mattermost/mattermost-server/v6/services/httpservice"
@@ -108,10 +107,6 @@ type Server struct {
 	goroutineCount      int32
 	goroutineExitSignal chan struct{}
 
-	PluginsEnvironment     *plugin.Environment
-	PluginConfigListenerId string
-	PluginsLock            sync.RWMutex
-
 	EmailService *email.Service
 
 	hubs     []*Hub
@@ -469,7 +464,7 @@ func NewServer(options ...Option) (*Server, error) {
 
 	})
 
-	s.telemetryService = telemetry.New(s, s.Store, s.SearchEngine, s.Log)
+	s.telemetryService = telemetry.New(New(ServerConnector(s.Channels())), s.Store, s.SearchEngine, s.Log)
 
 	emailService, err := email.NewService(email.ServiceConfig{
 		ConfigFn:           s.Config,
@@ -689,15 +684,6 @@ func NewServer(options ...Option) (*Server, error) {
 
 	s.initPostMetadata()
 
-	s.initPlugins(c, *s.Config().PluginSettings.Directory, *s.Config().PluginSettings.ClientDirectory)
-	s.AddConfigListener(func(prevCfg, cfg *model.Config) {
-		if *cfg.PluginSettings.Enable {
-			s.initPlugins(c, *cfg.PluginSettings.Directory, *s.Config().PluginSettings.ClientDirectory)
-		} else {
-			s.ShutDownPlugins()
-		}
-	})
-
 	// Dump the image cache if the proxy settings have changed. (need switch URLs to the correct proxy)
 	s.AddConfigListener(func(oldCfg, newCfg *model.Config) {
 		if (oldCfg.ImageProxySettings.Enable != newCfg.ImageProxySettings.Enable) ||
@@ -1002,17 +988,7 @@ func (s *Server) Shutdown() {
 
 	defer sentry.Flush(2 * time.Second)
 
-	// Stop products.
-	// This needs to happen before because products are dependent
-	// on parent services.
-	for name, product := range s.products {
-		if err := product.Stop(); err != nil {
-			mlog.Warn("Unable to cleanly stop product", mlog.String("name", name), mlog.Err(err))
-		}
-	}
-
 	s.HubStop()
-	s.ShutDownPlugins()
 	s.RemoveLicenseListener(s.licenseListenerId)
 	s.RemoveLicenseListener(s.loggerLicenseListenerId)
 	s.RemoveClusterLeaderChangedListener(s.clusterLeaderListenerId)
@@ -1096,6 +1072,15 @@ func (s *Server) Shutdown() {
 
 	mlog.Info("Server stopped")
 
+	// Stop products.
+	// This needs to happen last because products are dependent
+	// on parent services.
+	for name, product := range s.products {
+		if err2 := product.Stop(); err2 != nil {
+			mlog.Warn("Unable to cleanly stop product", mlog.String("name", name), mlog.Err(err2))
+		}
+	}
+
 	// shutdown main and notification loggers which will flush any remaining log records.
 	timeoutCtx, timeoutCancel := context.WithTimeout(context.Background(), time.Second*15)
 	defer timeoutCancel()
@@ -1204,6 +1189,14 @@ func stripPort(hostport string) string {
 }
 
 func (s *Server) Start() error {
+	// Start products.
+	// This needs to happen before because products are dependent on the HTTP server.
+	for name, product := range s.products {
+		if err := product.Start(); err != nil {
+			return errors.Wrapf(err, "Unable to start %s", name)
+		}
+	}
+
 	mlog.Info("Starting Server...")
 
 	var handler http.Handler = s.RootRouter
@@ -1406,14 +1399,6 @@ func (s *Server) Start() error {
 		mlog.Error("Error starting inter-cluster services", mlog.Err(err))
 	}
 
-	// Start products.
-	// This needs to happen after the server has started.
-	for name, product := range s.products {
-		if err := product.Start(); err != nil {
-			return errors.Wrapf(err, "Unable to start %s", name)
-		}
-	}
-
 	return nil
 }