Support a timestamp construction/alteration API in PyDough similar to the sqlite DATETIME function

[knassre-bodo]

This function should be able to do the following:

• Construct a datetime from its first argument which can be a datetime string, the word 'now' (meaning get the current timestamp), or a datetime literal (datetime.date, datetime.datetime, pandas.Timestamp)
• Use its subsequent arguments as modifiers:
  • Add/subtract year/month/day/hour/minute/second intervals by string: DATETIME('now', '-1 month', '+3 days'))
  • Truncates to start of the year, month, day in SQLite (or to the hour, minute, second in ANSI): DATETIME("now", "start of month")
• For SQlite, uses the sqlite datetime function accordingly
• For ANSI, uses a mix of CURRENT_DATE, CAST(... AS TIMESTAMP), ± INTERVAL 'x UNIT', and DATE_TRUNC