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/config.go

@@ -940,6 +940,7 @@ type ExperimentalSettings struct {
 	CloudUserLimit                  *int64  `access:"experimental,write_restrictable"`
 	CloudBilling                    *bool   `access:"experimental,write_restrictable"`
 	EnableSharedChannels            *bool   `access:"experimental"`
+	EnableRemoteClusterService      *bool   `access:"experimental"`
 }
 
 func (s *ExperimentalSettings) SetDefaults() {
@@ -979,6 +980,10 @@ func (s *ExperimentalSettings) SetDefaults() {
 	if s.EnableSharedChannels == nil {
 		s.EnableSharedChannels = NewBool(false)
 	}
+
+	if s.EnableRemoteClusterService == nil {
+		s.EnableRemoteClusterService = NewBool(false)
+	}
 }
 
 type AnalyticsSettings struct {