fix: Add raw datatype support for Db2 custom queries

[nj1973]

Description of changes

This PR solves an issue in our Db2 implementation where we couldn't get the raw data types for custom queries.

Describing the cursor did not reveal the actual data type, only the superset, e.g. string.

We solve the problem in the same way do on Redshift, by creating a view. If a view cannot be created then a warning is output and we fall back to the previous behaviour.

Issues to be closed

Closes #1691

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines