GQL Support Part 2: GQL Statements

[apstndb]

This PR implements the second phase of GQL support, focusing on GQL statements and refinements to the RETURN statement.

• part of Support Spanner Graph #98
• part of Split GQL related PRs #268

Implemented GQL Statements / Features

• GQL Statements
  • FILTER (including support for the optional WHERE keyword).
  • LET
  • FOR (including WITH OFFSET syntax).
  • ORDER BY (as a standalone statement, with ASCENDING/DESCENDING synonyms).
  • LIMIT (as a standalone statement).
  • OFFSET / SKIP (as a standalone statement).
  • WITH (supporting ALL/DISTINCT, *, AS alias, and GROUP BY with group hints).
• RETURN Statement Enhancements
  • Added support for trailing clauses: GROUP BY (with group hints), ORDER BY, OFFSET/SKIP, and LIMIT.
  • Enforced strict clause ordering: GROUP BY -> ORDER BY -> OFFSET/SKIP -> LIMIT.
• Syntax Rule Enforcement
  • Mandatory AS keyword for aliases in RETURN items.
  • Validation to ensure linear query segments end with a RETURN operator.

Unimplemented GQL Statements (Future Phases)

The following high-level features are planned for subsequent phases:

• MATCH statement.
• GQL Subqueries (EXISTS, ARRAY, VALUE, IN).
• CALL statement.
• GRAPH_TABLE operator (SQL integration).
• Undocumented or internal features, implemented in GoogleSQL reference implementation.