Use mbed::callback for BLE event handler typedefs on ARDUINO_ARCH_MBED

[DenebTM]

This allows using capturing lambdas for BLE device and characteristic event handlers on Mbed OS-based devices, as required by a recent project of mine:

for (auto tup : senswarn_chars) {
  auto ble_char = std::get<BLEUnsignedCharCharacteristic*>(tup);
  auto val_ptr  = std::get<bool*>(tup);

  ble_char->setEventHandler(
      BLEWritten,
      [val_ptr](BLEDevice central, BLECharacteristic characteristic) {
        bool warning_active = *characteristic.value() != 0;

        if (*val_ptr != warning_active) { *val_ptr = warning_active; }

        if (warning_active) { ch_any_warning_active.writeValue(true); }
      });

  sv_senswarn.addCharacteristic(*ble_char);
}

Note: Perhaps a plain std::function type is permissible here in general. I no longer have access to the hardware to test this, unfortunately.

[CLAassistant]

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