Initial investigations: Bluetooth LE

Subtask of #1 (closed). Related to briar#28, briar#303, briar#1147, briar#1546 (closed).

Investigate BLE advertising and scanning (including larger advertisement packets in BT 5), L2CAP connections, and coded PHY.

https://www.bluetooth.com/wp-content/uploads/2022/05/Bluetooth_LE_Primer_Paper.pdf