Support PostgreSQL conflict index elements, OVERRIDING USER and MERGE RETURNING

[minleejae]

PostgreSQL INSERT currently rejects conflict targets mixing columns and expressions, accepts only OVERRIDING SYSTEM VALUE, and MERGE has no RETURNING clause. This adds ordered conflict keys with COLLATE and schema-qualified operator classes, distinguishes USER/SYSTEM overriding, and connects MERGE to the existing ReturningClause (including OLD/NEW output aliases). Bare function keys and OVERRIDING without a column list are supported.

The target reuses Index.ColumnParams and centralizes expression traversal and rendering. Existing conflict-column getters remain mutable views, existing replacement methods remain available, and the boolean overriding API still maps true to SYSTEM. MERGE RETURNING uses visitor-based output; ExpressionDeParser now preserves normalized OLD/NEW qualifiers. Statement visitors, table discovery and validators visit the newly supported expressions.

Validation:

• Full Gradle check and Maven verify.
• Parsing, AST assertions, reparsing, deparser output, custom visitors and legacy mutation tests.
• Representative INSERT/MERGE statements executed on PostgreSQL 18.

References: PostgreSQL INSERT, PostgreSQL MERGE.