Remove Postgres compile time dependency and implement custom interval class

[brbrown25]

While working on #295 it was discovered that the PGInterval class does not allow for setting weeks as a constructor argument. Based on discussion here #295 (comment) we should remove the compile time dependency and implement a custom PGInterval that will allow for setting all supported parameters.

[brbrown25]

Of note, make_interval will convert weeks into a days representation, so we need to make sure we account for that. for example select make_interval(years => 1, months => 3, weeks => 2, days => 2, hours => 3, mins => 1, secs => 1.0); will return ``` make_interval

1 year 3 mons 16 days 03:01:01``` where 2 weeks = 14 days + 2 days to yield 16 days in the result
Similarly select make_interval(weeks => 1); will yield 7 days.

[brbrown25]

Gonna pick back up on this, this week.