Failure while parsing GIN Index

[LucaCappelletti94]

The parsing of the CREATE INDEX case, is currently incomplete. Here follows a couple of examples:

CREATE INDEX users_name_trgm_idx ON users USING gin (concat_users_name(first_name, last_name) gin_trgm_ops);

which raises:

CREATE INDEX projects_name_description_trgm_idx ON projects USING gin (concat_projects_name_description(name, description) gin_trgm_ops);: ParserError("Expected: ), found: gin_trgm_ops")

Or analogously:

CREATE INDEX sample_containers_barcode_trgm_idx ON sample_containers USING gin (barcode gin_trgm_ops);

which raises:

CREATE INDEX sample_containers_barcode_trgm_idx ON sample_containers USING gin (barcode gin_trgm_ops);: ParserError("Expected: ), found: gin_trgm_ops")

I will be attempting shortly to do a PR adding support for these use cases.

[LucaCappelletti94]

Right now, an Index only supports columns as defined by a OrderByExpr, which I do not believe is general enough to also handle the aforementioned cases. Is this correct?