feat: support GRAPH_TABLE operator

[apstndb]

Summary

• support GRAPH_TABLE as a table expression containing either an advanced GQL
  query block or the SQL/PGQ single-MATCH form;
• support the single-MATCH form both with an optional COLUMNS projection and
  without COLUMNS;
• support qualified graph names, joins, aliases, and TABLESAMPLE suffixes;
• add parser, unparser, position, walking, and round-trip goldens.

Note

Spanner's GoogleSQL dialect accepts an SQL/PGQ-style single-MATCH form
with optional COLUMNS, although this form is not currently documented in the
Spanner documentation. The upstream GoogleSQL compliance tests also cover this
grammar:

https://github.com/google/googlesql/blob/1f8aa333f4d6353cd3a64471fc83121df72df3f7/googlesql/compliance/testdata/graph_table.test

spanner> SELECT a_id FROM GRAPH_TABLE(FinGraph MATCH (a) COLUMNS(a.id AS a_id));
+------+
| a_id |
+------+
| 7    |
| 16   |
| 20   |
| 1    |
| 2    |
| 3    |
+------+
6 rows in set (9.03 msecs)

spanner> SELECT a.id FROM GRAPH_TABLE(FinGraph MATCH (a));
+----+
| id |
+----+
| 7  |
| 16 |
| 20 |
| 1  |
| 2  |
| 3  |
+----+
6 rows in set (8.82 msecs)

Related Issues

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