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

api4/channel.go

@@ -27,6 +27,7 @@ func (api *API) InitChannel() {
 
 	api.BaseRoutes.ChannelsForTeam.Handle("", api.ApiSessionRequired(getPublicChannelsForTeam)).Methods("GET")
 	api.BaseRoutes.ChannelsForTeam.Handle("/deleted", api.ApiSessionRequired(getDeletedChannelsForTeam)).Methods("GET")
+	api.BaseRoutes.ChannelsForTeam.Handle("/private", api.ApiSessionRequired(getPrivateChannelsForTeam)).Methods("GET")
 	api.BaseRoutes.ChannelsForTeam.Handle("/ids", api.ApiSessionRequired(getPublicChannelsByIdsForTeam)).Methods("POST")
 	api.BaseRoutes.ChannelsForTeam.Handle("/search", api.ApiSessionRequiredDisableWhenBusy(searchChannelsForTeam)).Methods("POST")
 	api.BaseRoutes.ChannelsForTeam.Handle("/search_archived", api.ApiSessionRequiredDisableWhenBusy(searchArchivedChannelsForTeam)).Methods("POST")
@@ -763,6 +764,32 @@ func getDeletedChannelsForTeam(c *Context, w http.ResponseWriter, r *http.Reques
 	w.Write([]byte(channels.ToJson()))
 }
 
+func getPrivateChannelsForTeam(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireTeamId()
+	if c.Err != nil {
+		return
+	}
+
+	if !c.App.SessionHasPermissionTo(*c.App.Session(), model.PERMISSION_MANAGE_SYSTEM) {
+		c.SetPermissionError(model.PERMISSION_MANAGE_SYSTEM)
+		return
+	}
+
+	channels, err := c.App.GetPrivateChannelsForTeam(c.Params.TeamId, c.Params.Page*c.Params.PerPage, c.Params.PerPage)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	err = c.App.FillInChannelsProps(channels)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	w.Write([]byte(channels.ToJson()))
+}
+
 func getPublicChannelsByIdsForTeam(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequireTeamId()
 	if c.Err != nil {