Improve support for ultra-low power scheduling, including hibernation via suspend/resume operations.

[dmitrykos]

Also:

• Add two C++ and C examples which demonstrate how to achieve ultra-low power scheduling based on STM32F407G-DISC1 board.

Examples are utilizing RTC timer for ultra-low power scheduling. User button demonstrates standby mode when STK is suspended on button press and resumed on a second press.

[github-actions]

LCOV of commit 98236b3 during Code Coverage - Generic #544

Summary coverage rate:
  lines......: 100.0% (1255 of 1255 lines)
  functions..: 42.8% (1575 of 3679 functions)
  branches...: no data found

Files changed coverage rate: n/a

[github-actions]

MemBrowse Memory Report

stm32f0-sleep

• FLASH: .text +76 B (+0.4%, 19,616 B / 65,536 B, total: 30% used)

stm32f1-sleep

• FLASH: .text +80 B (+0.4%, 18,737 B / 131,072 B, total: 14% used)

stm32f4-sleep

• FLASH: .text +84 B (+0.4%, 19,569 B / 1,048,576 B, total: 2% used)