feature: DATE_DIFF function

[BohuTANG]

Summary

Databend lacks a DATE_DIFF function, though it supports direct arithmetic on dates and times (e.g., TO_DATE(NOW()) - 2 for two days ago). However, users may find this conversion difficult, see: https://docs.databend.com/sql/sql-functions/datetime-functions/date-diff

Proposal add a DATE_DIFF function:

DATE_DIFF(<date_or_time_part>, <date_or_time_expr1>, <date_or_time_expr2>)

Similar to Snowflake's DATEDIFF function, this will simplify date and time arithmetic for users.

[chagelo]

I would like to have a try.

[chagelo]

1. Should select to_date('2024-02-29') + - to_date('2025-02-28'); be valid or invalid? I mean the add operation between two date.