Support For Multi-Row Inserts

[waltaskew]

The dialect currently doesn't support multi-row / bulk inserts, although Spanner itself does.

 engine = sa.create_engine(
 "spanner+spanner:///...", 
 )

 with engine.begin() as conn: 
 conn.execute(
 sa.insert(Person)
 .values([{"name": "jdoe"}, {"name": "jmoe"}])
 .returning(Person.id)
 )
 session.commit()

sqlalchemy.exc.CompileError: The 'spanner+spanner' dialect with current database version settings does not support in-place multirow inserts.