sql,plpgsql: add support for txn control statements in DO blocks

This issue tracks adding support for transaction control statements (`COMMIT`, `ROLLBACK`) within a `DO` statement. From the Postgres docs:
>In procedures invoked by the CALL command as well as in anonymous code blocks (DO command), it is possible to end transactions using the commands COMMIT and ROLLBACK.

Simple example:
```
postgres=# DO $$ BEGIN COMMIT; END $$;
DO
```

Jira issue: CRDB-46305