startAdvertising method
Serves the libp2p GATT service with identity (the local peer ID bytes)
in its identity characteristic, and advertises token.
A central that subscribes to TX opens a link: it arrives on incoming.
Implementation
@override
Future<void> startAdvertising(String token, Uint8List identity) async {
_token = token;
_identity = identity;
_advertising = true;
scheduleMicrotask(() => _network._announce(this));
}