Added 10 more date questions

[wongjingping]

Added 10 more date questions. 5 for derm_treatment and 5 for ewallet. The questions are designed to use different tables as much as possible, and are designed to have the following syntax:

derm_treatment:

• date diff date
• extract year window cte
• current date diff date
• text convert date diff date
• date null

ewallet:

• snapshot info
• left join on date overlap
• date diff date agg (avg)
• current date agg (pct)
• date diff date agg (min)