Returns the labels available in a mailbox: the built-in system labels — the placements `inbox`, `archive`, `spam`, `blocked`, and `sent`, plus `trash` and `unread` — followed by every custom label currently in use on its conversations and messages. Apply and remove labels through the conversation and message update endpoints; custom labels exist by being applied, so this list is discovery, not management. The label name, as it appears on conversations and messages. `system` labels are built in and carry state — the placements `inbox`, `archive`, `spam`, `blocked`, and `sent`, plus `trash` and `unread`. `custom` labels are the workspace's own tags. Possible values: `system`, `custom`