embassy-stm32: split rtc low-power wakeup and time functions

[tomhampshire]

Ref: #4110

The current rtc::Rtc struct combines low-power wakeup management and application-level time access (now(), set_datetime()). This creates an ownership conflict when embassy_stm32::low_power::stop_with_rtc requires exclusive control of the Rtc instance, preventing the application from simultaneously accessing time functions.

Changes to existing API have been minimised.

[tomhampshire]

Submitted for initial review - I don't yet have the hardware to test if the device is entering low-power states, so this still needs testing

[tomhampshire]

Example usage. The only change is the update to the configs (RtcConfig is now used as part of the global init) :

let mut config = embassy_stm32::Config::default();
let rtc_config = embassy_stm32::rtc::RtcConfig::default();
config.rtc = Some(rtc_config);
let p = embassy_stm32::init(config);

let rtc = embassy_stm32::rtc::Rtc::new(p.RTC);

[tomhampshire]

I have made an assumption that, if no RtcConfig is specified, the Rtc isn't configured. The other option is, if no RtcConfig is specified, the default config is used...

[tomhampshire]

@Dirbaio should we have the default behaviour be this (no RTC setup) or use the RtcConfig defaults?