Reflect index's column operator class on PostgreSQL

Reflect index's column operator class on PostgreSQL

Fill the `postgresql_ops` key of PostgreSQL's `dialect_options` returned by get_multi_indexes() with a mapping from column names to the operator class, if it's not the default for respective data type.

As we need to join on ``pg_catalog.pg_opclass``, the table definition is added to ``postgresql.pg_catalog``.

Fixes #8664.

Closes: #12504
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12504
Pull-request-sha: 8fdf93e1b27c371f52990d5fda8b2fdf79ec23eb

Change-Id: I8789c1e9d15f8cc9a7205f492ec730570f19bbcc