Consider using a template engine for code generation

[rbock]

A template engine like Jinja or Mako could be used for at least two use cases:

• sqlpp23-ddl2cpp code generation
• Generate header files to use custom types with sqlpp23 (very hand-wavy thought at this point).

Pros:

• Code generation code would be simpler.
• Library users could provide their own template with custom adjustments like additional includes.

Cons:

• New dependency.

[MeanSquaredError]

I just want to add another thought about the benefits of using a header template.

--custom-directives is a great feature but currently it adds the custom directives (or rather code) after all the include/import directives. In most cases it is OK, but in some rare cases order may matter. For example a few months ago when we were adding support for modules I had to reorder the includes quite a few times before I could find such an order that GCC 15 would compile sqlpp23 with modules support. Something similar could happen when --custom-directives alone would not be sufficient.