Add support for DML statements (INSERT/UPDATE/DELETE) as CTE bodies

[krleonid]

Allow INSERT, UPDATE, and DELETE statements with optional RETURNING clauses to be used as CTE bodies in WITH expressions. Previously, CTEs only supported SELECT statements.

Key changes:

• Widen CommonTableExpressionInfo::query from SelectStatement to SQLStatement to accept DML statement types alongside SELECT
• Add GetQueryNode() helper to uniformly access the root QueryNode regardless of the underlying statement type
• Transform DML statements (INSERT/UPDATE/DELETE) in CTE position during parsing, rejecting unsupported types like DDL and COPY
• Prevent recursive CTEs from using DML bodies (not semantically meaningful)
• Preserve DML CTE execution for side effects even when unreferenced, by skipping CTE inlining elimination for DML operators and force-binding unreferenced DML CTEs in the planner
• Update serialization with a new optional field (105: dml_query) for forward/backward compatible storage of DML CTE bodies
• Add comprehensive tests for DML CTEs, including unreferenced side-effect execution, error cases, and the previously failing issue INTERNAL Error: A CTE needs a SELECT duckdb/duckdb#3417

Made-with: Cursor