Use an internal new DataType DATETIME_NANOS which is not exposed
and therefore cannot be used for CASTing. DATETIME is used instead
and the precision of both DATETIME and TIME has been promoted from
3 to 9, providing transparency to all datetime functionality regardless
of millis or nanos precision.
Moreover, CURRENT_TIMESTAMP/CURRENT_TIME can now return precision up
to 6 fractional digits of a second with the use of Clock.
Closes: #38562
Co-authored-by: Bogdan Pintea <bogdan.pintea@elastic.co>
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