Add a BLE_NO_PIN mode or less secure BLE binding modes as an option

[arthurlutz]

In some use cases, having a PIN for bluetooth pairing can be a problem. It would be nice to be able to build a firmware (not necessarily an official firmware) with the default 123456 PIN disabled. And maybe have a choice in the authentication modes described in the docs : https://github.com/espressif/esp-idf/blob/master/examples/bluetooth/bluedroid/ble/gatt_security_server/tutorial/Gatt_Security_Server_Example_Walkthrough.md#setting-security-parameters

Context : I have an android device which doesn't manage to re-pair with existing paired device (bonds involved?), if it disconnects I have to go settings, forget the bluetooth device and then repair it (typing 123456 is driving me insane) .

[arthurlutz]

Trying out to do some code inspired by https://github.com/meshtastic/firmware/blob/fe288a70e4ce8697ef9934562ea8cb8eb0c443ff/src/nimble/NimbleBluetooth.cpp#L925-L930 and https://github.com/meshtastic/firmware/blob/fe288a70e4ce8697ef9934562ea8cb8eb0c443ff/src/nimble/NimbleBluetooth.cpp#L953-L961 in the draft PR #2948 ... but failing to make it work. If anyone else more competent than me in C++ can give it a shot, I'd be happy to test it out.