MM-23596: Ability to list private channels for team (#14925)

Summary:
store, app, api and go driver support for listing private channels

Ticket Link:
https://mattermost.atlassian.net/browse/MM-23596

app/app_iface.go

@@ -611,6 +611,7 @@ type AppIface interface {
 	GetPreferenceByCategoryForUser(userId string, category string) (model.Preferences, *model.AppError)
 	GetPreferencesForUser(userId string) (model.Preferences, *model.AppError)
 	GetPrevPostIdFromPostList(postList *model.PostList) string
+	GetPrivateChannelsForTeam(teamId string, offset int, limit int) (*model.ChannelList, *model.AppError)
 	GetProfileImage(user *model.User) ([]byte, bool, *model.AppError)
 	GetPublicChannelsByIdsForTeam(teamId string, channelIds []string) (*model.ChannelList, *model.AppError)
 	GetPublicChannelsForTeam(teamId string, offset int, limit int) (*model.ChannelList, *model.AppError)