Add Nexus slot supplier to WorkerTuner

The WorkerTuner currently supports slot suppliers for workflows, activities, and local activities. The bridge FFI layer (TemporalCoreTunerHolder) also has a nexus_slot_supplier field that is not yet exposed through the Swift API. When Nexus worker support is added, we should include a nexusSlotSupplier

Start with WorkerTuner and the TemporalCoreTunerHolder bridge, which already contains the nexus_slot_supplier field. Compare the existing workflow, activity, and local-activity slot supplier exposure to determine where nexusSlotSupplier belongs in the Swift API. Done means the Nexus slot supplier is exposed consistently with those existing suppliers.

Written by the indexing model from the issue text.