Postgres 13: Index opclassopts

Currently we error out if index has opclassopts.

Changelog entry on PG13:
Allow CREATE INDEX to specify the GiST signature length and maximum number of integer ranges (Nikita Glukhov)

The issue reports that indexes using opclassopts currently fail and cites PostgreSQL 13 support for GiST signature length and integer-range limits. Start by locating the C error path for index opclassopts and the related index tests; done means CREATE INDEX accepts the supported options without the current error and coverage verifies the behavior.

Written by the indexing model from the issue text.