date_part should support a nanosecond part #20671

Describe the bug

Trying to use nanosecond as a part with date_part results in an error. The documentation says it should work.

To Reproduce

Call the date_part function with the part specified as 'nanosecond'. e.g: (fiddle)

SELECT date_part('nanosecond', now());

Expected behavior

The result to be the number of seconds (including any fractional part * 1_000_000_000.

Additional context

To support this date_part() will have to return an Int64, rather than an Int32 if the part is 'nanosecond'. It already has special handling for 'epoch', so I can't see this being a problem.