Implement createIndex with column operator class

[EthanLozano]

Extend IndexItem to support an operator class.

Postgres allows indexes to specify an operator class. For instance, the pg_trgm extension could have the following index declaration:

CREATE INDEX trgm_idx ON test_trgm USING GIN (t gin_trgm_ops);

It appears that IndexItem.column(colum:order:) could have an "opclass" parameter for operator class configuration.

For the time being, I used an updateBuilder and raw sql to create an index with an operator class.