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