add new sketch to change the rtc mode

[FRASTM]

New application to demonstrate that changing the RTC mode from BIN to MIX and BCD does not affect the calendar nor the Alarm

Requires https://github.com/stm32duino/STM32RTC/pull/106

[FRASTM]

Output log:

12:08:13.010 -> RTC mode is MODE_BCD at 01/03/24 - 14:02:58 
12:08:15.000 -> Alarm A Match! : change RTC mode at 14:03:00 
12:08:15.001 -> RTC mode is MODE_MIX at 01/03/24 - 14:03:00 
12:09:15.005 -> Alarm A Match! : change RTC mode at 14:04:00 
12:09:15.005 -> RTC mode is MODE_BIN at 01/03/24 - 14:04:00 
12:10:14.992 -> Alarm A Match! : change RTC mode at 14:05:00 
12:10:14.992 -> RTC mode is MODE_BCD at 01/03/24 - 14:05:00 
...

[FRASTM]

rebase