support FOREIGN KEY without CONSTRAINTS clause

[nktks]

What

• support FOREIGN KEY without CONSTRAINT clause.
  • it is supported https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#create_table

and table_constraint is:
    [ CONSTRAINT constraint_name ]
    { CHECK ( expression ) |
      FOREIGN KEY ( column_name [, ... ] ) REFERENCES  ref_table  ( ref_column [, ... ] )
        [ ON DELETE { CASCADE | NO ACTION } ]

Background

• Since implement foreign key constraints #100 is merged, handy-spanner supports foreign key.
• But we met panic at below line when we use FOREIGN KEY clause without CONSTRAINT clause.
  • https://github.com/gcpug/handy-spanner/pull/100/files#diff-1ee14e92820fbbc3294d6aa06f24b6dc90b9119688e9ac82a677c87802860d18R1287

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1a6ec22]

goroutine 33 [running]:
testing.tRunner.func1.2({0x1d85d80, 0x305f670})
	/opt/hosted-tool-cache/go/1.21.10/x64/src/testing/testing.go:1545 +0x3f7
testing.tRunner.func1()
	/opt/hosted-tool-cache/go/1.21.10/x64/src/testing/testing.go:1548 +0x716
panic({0x1d85d80?, 0x305f670?})
	/opt/hosted-tool-cache/go/1.21.10/x64/src/runtime/panic.go:920 +0x270
github.com/gcpug/handy-spanner/server.(*database).registerInformationSchemaTables(0xc00069c500, {0x2308c80, 0x40d35e0}, 0xc000998180)
	/path/to/vendor/github.com/gcpug/handy-spanner/server/database.go:1290 +0x6e2
...

• When we use FOREIGN KEY without CONSTRAINTS clause, native spanner seems to insert autogenerated CONSTRAINT_NAME to INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS.

> spanner-cli -v
Connected.
spanner> CREATE TABLE Child
      -> (
      ->     ChildID STRING(MAX) NOT NULL,
      ->     ParentID STRING(MAX) NOT NULL,
      ->     FOREIGN KEY (ParentID) REFERENCES Parent (ParentID),
      -> ) PRIMARY KEY(ChildID);
Query OK, 0 rows affected (5.78 sec)

spanner> SELECT * FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS;
+--------------------+-------------------+------------------------------------+---------------------------+--------------------------+------------------------+--------------+-------------+-------------+---------------+
| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME                    | UNIQUE_CONSTRAINT_CATALOG | UNIQUE_CONSTRAINT_SCHEMA | UNIQUE_CONSTRAINT_NAME | MATCH_OPTION | UPDATE_RULE | DELETE_RULE | SPANNER_STATE |
+--------------------+-------------------+------------------------------------+---------------------------+--------------------------+------------------------+--------------+-------------+-------------+---------------+
|                    |                   | FK_Child_Parent_EDF8147E7630B0BA_1 |                           |                          | PK_Parent              | SIMPLE       | NO ACTION   | NO ACTION   | COMMITTED     |
+--------------------+-------------------+------------------------------------+---------------------------+--------------------------+------------------------+--------------+-------------+-------------+---------------+

• I'm not sure what value EDF8147E7630B0BA is, so for this case I insert fmt.Sprintf("FK_%s_%s_%d", tableName, cnst.ReferenceTable.Name, i) instead.

[double-di]

LGTM