Add support for PostgreSQL's ORDER BY ... USING <operator> clause

[LucaCappelletti94]

This PR adds parser and AST support for PostgreSQL USING operators in ORDER BY expressions, including:

• top-level ORDER BY
• aggregate argument list ORDER BY (e.g. inside aggfns(...) / string_agg(...) style clauses)

Problem

sqlparser-rs previously rejected valid PostgreSQL statements containing:

• ORDER BY <expr> USING <operator>

with errors like:

• Expected: ), found: using

This blocked parsing of several PostgreSQL regression-style aggregate statements.

Root Cause

parse_order_by_expr_inner only supported:

• ASC | DESC
• NULLS FIRST | NULLS LAST

and had no branch to parse USING <operator>.

Dialect Scope Decision

ORDER BY ... USING <operator> is intentionally enabled for PostgreSQL only.

Rationale:

• PostgreSQL explicitly documents ORDER BY expression [ USING operator ] ... in SELECT.
• Other dialects supported by this repository generally document ORDER BY with expression + ASC|DESC (+ null ordering), but not USING <operator>.

References:

• PostgreSQL: https://www.postgresql.org/docs/current/sql-select.html
• Redshift: https://docs.aws.amazon.com/redshift/latest/dg/r_ORDER_BY_clause.html
• Snowflake: https://docs.snowflake.com/en/sql-reference/constructs/order-by
• BigQuery: https://cloud.google.com/bigquery/docs/reference/standard-sql/query-syntax
• MySQL: https://dev.mysql.com/doc/refman/8.0/en/query-expression.html
• SQLite: https://www.sqlite.org/lang_select.html
• DuckDB: https://duckdb.org/docs/stable/sql/query_syntax/orderby.html
• Oracle: https://docs.oracle.com/en/database/oracle/oracle-database/26/sqlrf/SELECT.html
• SQL Server: https://learn.microsoft.com/sql/t-sql/queries/select-order-by-clause-transact-sql

Complete Examples

PostgreSQL: accepted

SELECT a FROM t ORDER BY a USING <;

SELECT a FROM t ORDER BY a USING OPERATOR(pg_catalog.<) NULLS LAST;

SELECT aggfns(DISTINCT a, a, c ORDER BY c USING ~<~, a)
FROM (VALUES (1,3,'foo'),(0,NULL,NULL),(2,2,'bar'),(3,1,'baz')) v(a,b,c),
     generate_series(1,2) i;

CREATE OR REPLACE VIEW agg_view1 AS
SELECT aggfns(a,b,c ORDER BY c USING ~<~)
FROM (VALUES (1,3,'foo'),(0,NULL,NULL),(2,2,'bar'),(3,1,'baz')) v(a,b,c);

PostgreSQL: rejected (invalid syntax)

SELECT a FROM t ORDER BY a USING ;
-- Error: Expected an ordering operator after USING

SELECT a FROM t ORDER BY a USING OPERATOR();
-- Error: Expected an operator name

SELECT a FROM t ORDER BY a USING < DESC;
-- Error: ASC/DESC cannot be used together with USING in ORDER BY