Support Postgresql INTERVAL fields spec/reflection

Support Postgresql INTERVAL fields spec/reflection

Added support for all possible "fields" identifiers when reflecting the
Postgresql ``INTERVAL`` datatype, e.g. "YEAR", "MONTH", "DAY TO
MINUTE", etc..   In addition, the :class:`.postgresql.INTERVAL`
datatype itself now includes a new parameter
:paramref:`.postgresql.INTERVAL.fields` where these qualifiers can be
specified; the qualifier is also reflected back into the resulting
datatype upon reflection / inspection.

Change-Id: I33816e68c533b023e0632db6f4e73fefd2de4721
Fixes: #3959