[YSQL] Support ASC/DESC/HASH on GIN indexes

[jaki]

Jira Link: DB-987

Description

GIN indexes currently do not support user-supplied ASC/DESC/HASH on index column. They default to ASC because that's what enables tsvector prefix match to work. But this isn't relevant if users don't care about tsvector prefix match.

Allow ASC/DESC/HASH to be used on GIN indexes. It should have no consequences when done on types like arrays or jsonbs. Even for tsvector, if there's a way to figure out and resort to full scan in case the index is HASH and is using prefix match, then that works.