Use Jinja2 for C++ code generation in sqlpp23-ddl2cpp

[rbock]

The sqlpp23-ddl2cpp script was refactored to use Jinja2 for C++ code generation instead of manual print statements. A unified template was introduced in the ModelWriter class to handle headers, split headers, and modules. The script was verified against the existing unit test suite, and whitespace control was fine-tuned to ensure consistent formatting in the generated C++ files.

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PR created automatically by Jules for task 14377198940802934772 started by @rbock

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[MeanSquaredError]

I just browsed quickly through the PR and didn't see any obvious issues. Actually it looks quite good to me!

I think that this PR doesn't add an option to specify a custom template, or at least I did not see one, but that's OK since it can be added in a separate PR.

I just want to mention that besides an option to specify the custom template (e.g. --path-to-custom-tpl), it would be nice to have an option to see the built-in default template (e.g. --show-default-tpl). This would allow the user to save the text of the default template (by redirecting the output of ddl2cpp), to modify it with a text editor and then use the modified template as a custom one.

[rbock]

At this point, I am not sure I will go with this option yet. But it is a good basis for discussion.

[google-labs-jules]

At this point, I am not sure I will go with this option yet. But it is a good basis for discussion.

Acknowledged. I'm glad to hear it's a helpful starting point for your evaluation. Please let me know if you'd like any specific adjustments or further information regarding this Jinja2 refactoring.