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/platform/services/sharedchannel/sync_send_remote.go

@@ -527,13 +527,17 @@ func (scs *Service) sendProfileImageSyncData(sd *syncData) {
 	}
 }
 
-// sendSyncMsgToRemote synchronously sends the sync message to the remote cluster.
+// sendSyncMsgToRemote synchronously sends the sync message to the remote cluster (or plugin).
 func (scs *Service) sendSyncMsgToRemote(msg *model.SyncMsg, rc *model.RemoteCluster, f sendSyncMsgResultFunc) error {
 	rcs := scs.server.GetRemoteClusterService()
 	if rcs == nil {
 		return fmt.Errorf("cannot update remote cluster %s for channel id %s; Remote Cluster Service not enabled", rc.Name, msg.ChannelId)
 	}
 
+	if rc.PluginID != "" {
+		return scs.sendSyncMsgToPlugin(msg, rc, f)
+	}
+
 	b, err := json.Marshal(msg)
 	if err != nil {
 		return err
@@ -568,6 +572,17 @@ func (scs *Service) sendSyncMsgToRemote(msg *model.SyncMsg, rc *model.RemoteClus
 	return err
 }
 
+// sendSyncMsgToRemote synchronously sends the sync message to a plugin.
+func (scs *Service) sendSyncMsgToPlugin(msg *model.SyncMsg, rc *model.RemoteCluster, f sendSyncMsgResultFunc) error {
+	syncResp, errResp := scs.app.OnSharedChannelsSyncMsg(msg, rc)
+
+	if f != nil {
+		f(syncResp, errResp)
+	}
+
+	return errResp
+}
+
 func sanitizeSyncData(sd *syncData) {
 	for id, user := range sd.users {
 		sd.users[id] = sanitizeUserForSync(user)