Verify that isolation levels work as expected with PGAdapter

[olavloite]

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.