Shared channels plugin APIs for MS Teams plugin (#25805)

New plugin APIs and hooks for accessing Shared Channels service via plugin. 

- RegisterPluginForSharedChannels(opts model.RegisterPluginOpts) (remoteID string, err error)
- UnregisterPluginForSharedChannels(pluginID string) error
- ShareChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UpdateSharedChannel(sc *model.SharedChannel) (*model.SharedChannel, error)
- UnshareChannel(channelID string) (unshared bool, err error)
- UpdateSharedChannelCursor(channelID, remoteID string, cusror model.GetPostsSinceForSyncCursor) error
- SyncSharedChannel(channelID string) error
- InviteRemoteToChannel(channelID string, remoteID string, userID string) error
- UninviteRemoteFromChannel(channelID string, remoteID string) error

Hooks
- OnSharedChannelsSyncMsg(msg *model.SyncMsg, rc *model.RemoteCluster) (model.SyncResponse, error)
- OnSharedChannelsPing(rc *model.RemoteCluster) bool

server/channels/app/server.go

@@ -625,8 +625,8 @@ func (s *Server) startInterClusterServices(license *model.License) error {
 	}
 
 	var err error
-
-	rcs, err := remotecluster.NewRemoteClusterService(s)
+	appInstance := New(ServerConnector(s.Channels()))
+	rcs, err := remotecluster.NewRemoteClusterService(s, appInstance)
 	if err != nil {
 		return err
 	}
@@ -653,7 +653,6 @@ func (s *Server) startInterClusterServices(license *model.License) error {
 		return nil
 	}
 
-	appInstance := New(ServerConnector(s.Channels()))
 	scs, err := sharedchannel.NewSharedChannelService(s, appInstance)
 	if err != nil {
 		return err