feat(ios): add alternativeAdvertisingNameEnabled option for DFU

Add new DfuOptions.alternativeAdvertisingNameEnabled to control whether the iOS
DFU library sends an alternative advertising name to the device before
switching to bootloader mode. The library defaults this to true.

Context: the micro:bit V2 bootloader is compiled with NRF_DFU_BLE_REQUIRES_BONDS=1,
which compiles out the code path that reads a stored advertising name. The bootloader
accepts the name via SVCI (opcode 0x02 returns Success) but always advertises as
"DfuTarg". With the alternative name enabled, the DFU library scans for the wrong
name and times out. Setting this to false makes the library scan by DFU service UUID
instead, connecting to the first matching device.

Author: microbit-matt-hillsdon