Convert PathKey to use CompareType

Convert PathKey to use CompareType

Change the PathKey struct to use CompareType to record the sort
direction instead of hardcoding btree strategy numbers.  The
CompareType is then converted to the index-type-specific strategy when
the plan is created.

This reduces the number of places btree strategy numbers are
hardcoded, and it's a self-contained subset of a larger effort to
allow non-btree indexes to behave like btrees.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com