FEATURE: add_days() helper

[RasmusSkytte]

Describe the feature
This will be a age helper like add_years() but which adds days to a date

What use will this feature have?
This will allow us to more transparently write feature stores for backends which have poor date support (e.g. SQL Server).
On backends such as this, we unfortunately cannot just write date + 1 to get the next date.

Having an add_days() helper means that we can write add_days(date, 1) to get the next date in a transparent manor.

Who should add the feature?
N/A

Additional context
N/A

[RasmusSkytte]

When this gets implemented, we can add SQL server support to DiseasystoreSimulist