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)

shared/mlog/levels.go

@@ -30,6 +30,18 @@ var (
 	// used by the TCP log target
 	LvlTcpLogTarget = LogLevel{ID: 120, Name: "TcpLogTarget"}
 
+	// used by Remote Cluster Service
+	LvlRemoteClusterServiceDebug = LogLevel{ID: 130, Name: "RemoteClusterServiceDebug"}
+	LvlRemoteClusterServiceError = LogLevel{ID: 131, Name: "RemoteClusterServiceError"}
+	LvlRemoteClusterServiceWarn  = LogLevel{ID: 132, Name: "RemoteClusterServiceWarn"}
+
+	// used by Shared Channel Sync Service
+	LvlSharedChannelServiceDebug            = LogLevel{ID: 200, Name: "SharedChannelServiceDebug"}
+	LvlSharedChannelServiceError            = LogLevel{ID: 201, Name: "SharedChannelServiceError"}
+	LvlSharedChannelServiceWarn             = LogLevel{ID: 202, Name: "SharedChannelServiceWarn"}
+	LvlSharedChannelServiceMessagesInbound  = LogLevel{ID: 203, Name: "SharedChannelServiceMsgInbound"}
+	LvlSharedChannelServiceMessagesOutbound = LogLevel{ID: 204, Name: "SharedChannelServiceMsgOutbound"}
+
 	// add more here ...
 )