feat(postgresql): Add star expander for SELECT * and RETURNING *

[kyleconroy]

Summary

Adds a new expander package under internal/engine/postgresql/ that expands * expressions in SQL queries to explicit column names by preparing the query against a PostgreSQL database.

• Expands SELECT * to explicit column list
• Preserves table prefix for qualified stars (e.g., authors.* → authors.id, authors.name, authors.bio)
• Handles RETURNING * in INSERT/UPDATE/DELETE statements
• Recursively expands CTEs, including dependent CTEs where later CTEs reference earlier ones
• Supports subqueries in FROM clause
• Works with both cgo (pganalyze/pg_query_go) and non-cgo (wasilibs/go-pgquery) builds

Example

-- Input
SELECT * FROM authors

-- Output
SELECT id, name, bio FROM authors

-- Input
WITH a AS (SELECT * FROM authors), b AS (SELECT * FROM a) SELECT * FROM b

-- Output
WITH a AS (SELECT id, name, bio FROM authors), b AS (SELECT id, name, bio FROM a) SELECT id, name, bio FROM b

Test plan

• Unit tests for all supported query types
• Tests for CTEs with dependencies
• Tests for table-qualified stars
• Builds with both cgo and non-cgo

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