MM-43145: channels loader (#19957)

Add channels dataloader

https://mattermost.atlassian.net/browse/MM-43145

```release-note
NONE
```

app/app_iface.go

@@ -569,6 +569,7 @@ type AppIface interface {
 	GetChannelPinnedPostCount(channelID string) (int64, *model.AppError)
 	GetChannelPoliciesForUser(userID string, offset, limit int) (*model.RetentionPolicyForChannelList, *model.AppError)
 	GetChannelUnread(channelID, userID string) (*model.ChannelUnread, *model.AppError)
+	GetChannels(channelIDs []string) ([]*model.Channel, *model.AppError)
 	GetChannelsByNames(channelNames []string, teamID string) ([]*model.Channel, *model.AppError)
 	GetChannelsForRetentionPolicy(policyID string, offset, limit int) (*model.ChannelsWithCount, *model.AppError)
 	GetChannelsForScheme(scheme *model.Scheme, offset int, limit int) (model.ChannelList, *model.AppError)

app/channel.go

@@ -1736,6 +1736,20 @@ func (s *Server) getChannel(channelID string) (*model.Channel, *model.AppError)
 	return channel, nil
 }
 
+func (a *App) GetChannels(channelIDs []string) ([]*model.Channel, *model.AppError) {
+	channels, err := a.Srv().Store.Channel().GetMany(channelIDs, true)
+	if err != nil {
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(err, &nfErr):
+			return nil, model.NewAppError("GetChannel", "app.channel.get.existing.app_error", nil, nfErr.Error(), http.StatusNotFound)
+		default:
+			return nil, model.NewAppError("GetChannel", "app.channel.get.find.app_error", nil, err.Error(), http.StatusInternalServerError)
+		}
+	}
+	return channels, nil
+}
+
 func (a *App) GetChannelByName(channelName, teamID string, includeDeleted bool) (*model.Channel, *model.AppError) {
 	var channel *model.Channel
 	var err error

app/opentracing/opentracing_layer.go

@@ -5176,6 +5176,28 @@ func (a *OpenTracingAppLayer) GetChannelUnread(channelID string, userID string)
 	return resultVar0, resultVar1
 }
 
+func (a *OpenTracingAppLayer) GetChannels(channelIDs []string) ([]*model.Channel, *model.AppError) {
+	origCtx := a.ctx
+	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.GetChannels")
+
+	a.ctx = newCtx
+	a.app.Srv().Store.SetContext(newCtx)
+	defer func() {
+		a.app.Srv().Store.SetContext(origCtx)
+		a.ctx = origCtx
+	}()
+
+	defer span.Finish()
+	resultVar0, resultVar1 := a.app.GetChannels(channelIDs)
+
+	if resultVar1 != nil {
+		span.LogFields(spanlog.Error(resultVar1))
+		ext.Error.Set(span, true)
+	}
+
+	return resultVar0, resultVar1
+}
+
 func (a *OpenTracingAppLayer) GetChannelsByNames(channelNames []string, teamID string) ([]*model.Channel, *model.AppError) {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.GetChannelsByNames")