feat(ble): expose descriptor operations (discoverDescriptors + writeDescriptor)

[Zcating]

iOS Core Bluetooth requires explicit discoverDescriptors before setNotifyValue can find the CCCD (0x2902) descriptor handle. Without these primitives exposed, callers cannot enable BLE notifications on iOS — the subscription is a silent no-op.

Changes:

• specs/NativeBleNitro.nitro.ts: add 2 new spec methods
• manager.ts: add Promise-returning wrappers that normalize UUIDs
• ios/BleNitroBleManager.swift: implement discoverDescriptors/writeDescriptor using CBPeripheral.discoverDescriptors + writeValue(for: descriptor)
• ios/BlePeripheralDelegate.swift: store per-descriptor callbacks, fire them from didDiscoverDescriptorsFor and didWriteValueFor(error:)
• android/BleNitroBleManager.kt: implement both methods (Android discovers descriptors as part of characteristic discovery, so discoverDescriptors is a fast no-op resolving once cached descriptors are available; writeDescriptor reuses the existing GattOperation.WriteDescriptor queue)