MM-16261: Synchronize plugins in HA (#11657)

* MM-16272 - Synchronize plugins across cluster (#11611)

* MM-16272 - Synchronize plugins across cluster

* Adding a test

* MM-16272 - Fixed tests

* MM-16272 - PR feedback

* MM-16270 - Plugin Sync (#11615)

* Initial implementation for plugin synch with file store. WIP

* Removed ListAll implementation. Used ListDirectory and change localstore to be consistent and return all items (files and folders) from directory

* Refactored plugin filestore operations out of main install/remove plugin

* Fixing error handling details

* Changes to use structured logging

* More logging fixes

* Wording and comments improvements

* Error handling and control flow improvements

* Changed managed flag check to use os.stat

* Added file store plugin dir and filename consts

* Replaced FileRead to use a the FileReader in PluginSync

* Minor styling and PR feedback changes

* Minor error handling improvements

* Added unit test for SyncPlugins. Changed SyncPlugins to use plugins environment to list available plugins

* PR Feedback improvements

* Minor err handling fix

* Removing FileStorePath from PluginEventData (#11644)

* Fix plugin path (#11654)

* tweak path, logging

Fix an issue not finding the plugins folder in S3. Tweak logging messages to add additional clarity.

* Removing FileExists check when Syncing plugins. Updated localstore to not return an error when directory does not exist

* PR Feedback

* Install prepackaged plugins locally only (#11656)

* s/uninstall/remove

* Updated ClusterMessage comment

* Updated PluginSync to test against s3 + local storage

app/plugin_event.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"github.com/mattermost/mattermost-server/model"
+)
+
+// notifyClusterPluginEvent publishes `event` to other clusters.
+func (a *App) notifyClusterPluginEvent(event string, data model.PluginEventData) {
+	if a.Cluster != nil {
+		a.Cluster.SendClusterMessage(&model.ClusterMessage{
+			Event:            event,
+			SendType:         model.CLUSTER_SEND_RELIABLE,
+			WaitForAllToSend: true,
+			Data:             data.ToJson(),
+		})
+	}
+}