MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

model/feature_flags.go

@@ -19,6 +19,12 @@ type FeatureFlags struct {
 	// Toggle on and off support for Collapsed Threads
 	CollapsedThreads bool
 
+	// Enable the remote cluster service for shared channels.
+	EnableRemoteClusterService bool
+
+	// Toggle on and off support for Custom User Statuses
+	CustomUserStatuses bool
+
 	// AppsEnabled toggle the Apps framework functionalities both in server and client side
 	AppsEnabled bool
 
@@ -37,6 +43,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.TestBoolFeature = false
 	f.CloudDelinquentEmailJobsEnabled = false
 	f.CollapsedThreads = false
+	f.EnableRemoteClusterService = false
 	f.FilesSearch = false
 	f.AppsEnabled = false