Add command line tools

[colemancda]

Summary

Adds five executable targets for interacting with Bluetooth controllers on Linux, built on the existing HCI, L2CAP, and GATT support:

• hcitool — list controllers (dev), classic inquiry (inq), and Low Energy scanning (lescan) with advertised local name decoding.
• hciconfig — print controller information (list) and bring controllers up or down over the existing ioctl wrappers.
• gatttool — GATT client for LE peripherals: discover primary services and characteristics, read and write characteristic values by handle or UUID, and subscribe to notifications and indications.
• gattserver — advertises a configurable local name and serves a demo GATT database (Device Information and Battery services), re-listening after each disconnect.
• beacon — advertise as an iBeacon (ibeacon) and stop advertising (stop).

Adds swift-argument-parser as a dependency for the new executable targets only; the BluetoothLinux library itself is unchanged.

Test plan

• swift build succeeds with no warnings in the new targets
• Existing test suite passes (40 tests)
• Argument parsing verified for all subcommands via --help
• Runtime behavior requires Linux with a Bluetooth adapter and root privileges