Add RTC_DS3231::{get,set}Aging() to access the Aging Offset Register.

[dpwe]

RTClib provides access to almost all the functions of the DS3231 RTC except the Aging Offset register.

I've had good results using this register to trim my DS3231 modules against a GPS clock reference (see https://dpwe.github.io/arduinoclocks/2022-03-20-synchronizer.html). So it's nice to be able to access the Aging Offset register within the RTClib API.