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/store/timerlayer/timerlayer.go

@@ -6905,6 +6905,22 @@ func (s *TimerLayerRemoteClusterStore) GetAll(filter model.RemoteClusterQueryFil
 	return result, err
 }
 
+func (s *TimerLayerRemoteClusterStore) GetByPluginID(pluginID string) (*model.RemoteCluster, error) {
+	start := time.Now()
+
+	result, err := s.RemoteClusterStore.GetByPluginID(pluginID)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("RemoteClusterStore.GetByPluginID", success, elapsed)
+	}
+	return result, err
+}
+
 func (s *TimerLayerRemoteClusterStore) Save(rc *model.RemoteCluster) (*model.RemoteCluster, error) {
 	start := time.Now()