OF-2537: Advertise support for pubsub's 'multi-item' feature XEP-0060 defines, amongst many others, the 'multi-items' service discovery feature.
This feature can be advertised by a pubsub service, when clients are allowed to configure a node in such a way that it can accept more-than-one item, by:
- setting `persist_items` to `true`
- setting `max_items` to a value higher than 1.
