Verify that isolation levels work as expected with PGAdapter

Check that:

1. The documentation regarding isolation levels are up to date (both GitHub and https://docs.cloud.google.com/spanner/docs/pgadapter-session-mgmt-commands)
2. Setting a default transaction isolation level works (also using the PG session properties `default_transaction_isolation` and `transaction_isolation`).
3. Showing the current isolation level shows the correct value.