Support Spanner Graph

[apstndb]

Spanner Graph introduces ISO GQL and SQL/PGQ subsets to Cloud Spanner. It is coexists with queries and DDLs of GoogleSQL, so I think it is better to be implemented by memefish.

• {CREATE | DROP} PROPERTY GRAPH DDL statements

• GRAPH_TABLE operator

  SELECT name, id
  FROM GRAPH_TABLE(
  FinGraph
  MATCH (n)
  RETURN n.name AS name, n.id AS id
  );

• IN { gql } syntax

   -- Use Graph pattern matching to identify Account nodes to update:
  UPDATE Account SET is_blocked = false
  WHERE id IN {
  GRAPH FinGraph
  MATCH (a:Account WHERE a.id = 1)-[:TRANSFERS]->{1,2}(b:Account)
  RETURN b.id
  }

• Top level GQL query syntax