bug: Support Retrieval of Cross-Schema Foreign Keys

Allow `get_multi_foreign_keys` to retrieve foreign keys between tables
in different schemas. The current SQL used to retrieve them has a
mistake in the JOINs used to retrieve them, confusing the `table_schema`
column and the `constraint_schema` column. This goes unnoticed as long
as everything is in the same schema.

fixes: #638

Author: waltaskew

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -1472,10 +1472,12 @@ def get_multi_foreign_keys(
             )
             FROM information_schema.table_constraints AS tc
             JOIN information_schema.constraint_column_usage AS ccu
-                USING (table_catalog, table_schema, constraint_name)
+                ON ccu.table_catalog = tc.table_catalog
+                and ccu.constraint_schema = tc.table_schema
+                and ccu.constraint_name = tc.constraint_name
             JOIN information_schema.constraint_table_usage AS ctu
                 ON ctu.table_catalog = tc.table_catalog
-                and ctu.table_schema = tc.table_schema
+                and ctu.constraint_schema = tc.table_schema
                 and ctu.constraint_name = tc.constraint_name
             JOIN information_schema.key_column_usage AS kcu
                 ON kcu.table_catalog = tc.table_catalog