Feature: Allow extract parts from interval type

[rad-pat]

Summary
Similar to EXTRACT from timestamp, allow EXTRACT from an interval to give the various parts. For interval type,

EPOCH == total number of seconds
DAY == number of days
HOUR == number of hours
MINUTE == number of minutes
SECOND == number of seconds
MICROSECONDS == seconds * 1,000,000

e.g.

select  EXTRACT(day from '1 day 2 hours 3 minutes'::INTERVAL)
93780

select  EXTRACT(day from '1 day 2 hours 3 minutes'::INTERVAL)
1

select  EXTRACT(hour from '1 day 2 hours 3 minutes'::INTERVAL)
2

select  EXTRACT(minute from '1 day 2 hours 3 minutes'::INTERVAL)
3

SELECT EXTRACT(second from '1 day 02:03:04.1234'::INTERVAL);
4.1234

SELECT EXTRACT(microsecond from '1 day 02:03:04.1234'::INTERVAL);
4123400