feat(spanner/spansql): Add tests and parser support for THEN RETURN

[c2nes]

This change adds THEN RETURN support for DML statements. The existing "Query" methods already support executing such statements, so this change is limited to updating documentation, adding integration tests, and updating the spansql package. Support has not been added to the spannertest in-memory implementation of Spanner, but the feature absence has been noted in the corresponding README.

[c2nes]

Note: Integration tests are not expected to pass until THEN RETURN support is launched.