Check for holidays

[dovregubben]

The cron expression does not allow for checking of holidays.
This update allows the lambda function to optionally check if the current date is a holiday - and if so, stops running without invoking any schedule.

Makes use of python package holidays, see official docs.
holidays and dependencies were put into the packages folder by issueing
pip install --target ./package holidays.

Major implementation in main.py.
Furthermore, two optional variables were introduced for handling the new functionality:

• SCHEDULE_DISABLE_HOLIDAYS: bool flag to determine whether check for holidays should be conducted
• SCHEDULE_HOLIDAYS_COUNTRY: country ISO code for holidays