Fixes: https://github.com/espressif/arduino-esp32/issues/5573
To reproduce:
1. Run any sample code that starts a BLE server, and does not call `setCallbacks`.
2. Connect to the device using the "LightBlue" app on iOS.
3. Observe crash shown in the issue linked above.
* add onMtuChanged to BLEServerCallbacks
Add method onMtuChanged to the BLEServerCallbacks so the application can be notified if the MTU size changes.
* Correct missing semicolon and misspelled method.
Correct missing semicolon and misspelled method.
* Replaced ARDUINO_VARIANT with const char
* Fixed missing return value
* Added quotes around defined value in macro (Issue #2193)
* Change logging from Error to Verbose when not found and default available
* Move Enter and Exit logging to Verbose Level
* Refactored LOG_X() into log_x()