Make it possible to run a custome SQL command from within a test

### Feature Description

Add (a) method(s) to `DBTestSuite` that will allow to execute a custom SQL code as part of the test.

(no need for parametrized queries support, at least yet)

### Problem / Opportunity

Sometimes a database test needs to run a different SQL than just interaction with table or calling a function.

### Acceptance Criteria

1. User of the library can run a custom SQL

### Proposed Solution

Add methods:
* `sql` that returns a result set, similar to table query or function call
* `ddl` that returns no results
* (possible) `dml`, that allows to modify data, returning the rows modified
* auto-adding RETURNING, if not present
* possible to call with verification as table query, or without

### Dependencies / Related

_No response_

### Additional Context

_No response_