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

store/store.go

@@ -156,6 +156,7 @@ type ChannelStore interface {
 	GetAllChannels(page, perPage int, opts ChannelSearchOpts) (*model.ChannelListWithTeamData, error)
 	GetAllChannelsCount(opts ChannelSearchOpts) (int64, error)
 	GetMoreChannels(teamId string, userId string, offset int, limit int) (*model.ChannelList, error)
+	GetPrivateChannelsForTeam(teamId string, offset int, limit int) (*model.ChannelList, *model.AppError)
 	GetPublicChannelsForTeam(teamId string, offset int, limit int) (*model.ChannelList, *model.AppError)
 	GetPublicChannelsByIdsForTeam(teamId string, channelIds []string) (*model.ChannelList, *model.AppError)
 	GetChannelCounts(teamId string, userId string) (*model.ChannelCounts, *model.AppError)