fix: support THEN RETURN for insert, update, delete

[olavloite]

Support THEN RETURN clauses for INSERT, UPDATE, and DELETE statements.

Without this change, inserting a value into a table that uses a bit-reversed sequence for generating a primary key looks like this:

2024-11-26 18:35:31,309 INFO sqlalchemy.engine.Engine SELECT GET_NEXT_SEQUENCE_VALUE(SEQUENCE ticket_sale_id)
2024-11-26 18:35:31,309 INFO sqlalchemy.engine.Engine [raw sql] []
2024-11-26 18:35:31,312 INFO sqlalchemy.engine.Engine INSERT INTO ticket_sales (id, customer_name, seats, venue_code, start_time, singer_id) VALUES (%s, %s, %s, %s, %s, %s)
2024-11-26 18:35:31,312 INFO sqlalchemy.engine.Engine [generated in 0.00307s] [4611686018427387904, 'Alice Doe', ['A010', 'A011', 'A012'], 'CH', datetime.datetime(2024, 11, 7, 19, 30), '13cd9756-103a-48ec-97c1-03dd32540120']

With this change, the same example looks like this:

2024-11-26 18:37:48,981 INFO sqlalchemy.engine.Engine INSERT INTO ticket_sales (id, customer_name, seats, venue_code, start_time, singer_id) VALUES ( GET_NEXT_SEQUENCE_VALUE(SEQUENCE ticket_sale_id), %s, %s, %s, %s, %s) THEN RETURN ticket_sales.id
2024-11-26 18:37:48,981 INFO sqlalchemy.engine.Engine [generated in 0.00008s] ['Alice Doe', ['A010', 'A011', 'A012'], 'CH', datetime.datetime(2024, 11, 7, 19, 30), '40879c57-a640-4652-8d35-ea0f6970ff85']

In other words: 1 round-trip less to Spanner.

Fixes #498