Support TIMESTAMP_NS, TIMESTAMP_MS & TIMESTAMP_S types

[JelteF]

DuckDB has timestamp types with different precisions. We should support conversion from/to those types. TIMESTAMP_NS is probably the most challenging, since postgres does not have a native timestamp type with nanosecond precision. It seems better to truncate/round the nanoseconds to microseconds though than to not support them at all.

Related to #228, which tracks adding TIMESTAMP WITH TIME ZONE.