Use `next_item` CTE helper for sled subnet allocation when adding a sled to an initialized rack

[andrewjstone]

Currently, subnet allocation is done manually in omicron_nexus::app::rack::Nexus::add_sled_to_initialized_rack inside rust code, outside CRDB. However, this means that concurrent add sled operations will fail due to reusing allocations. We can avoid this problem via the use of the next_item helper