GH-1061: Add codegen classifier jar for arrow-vector.

[lriggs]

What's Changed

Add a new codegen classifier jar for arrow-vector that contains tdd and other template files.

Closes #1061 .

[lidavidm]

Can you explain how these are useful?

[telemenar]

Can you explain how these are useful?

Arrow's type system has many types, and projects building operations across Arrow data (comparisons, casts, aggregations) benefit from generating type-specialized code rather than hand-writing implementations for each type.

The TDD files provide a machine-readable definition of Arrow's types that enables this. Including them in the distribution allows downstream projects to generate code that stays in sync as Arrow's type system evolves.

[lidavidm]

Thanks! Can this rationale be documented in the code itself?

[lriggs]

I added a comment about the usefulness of the codegen files.

[lidavidm]

Ah, I would've expected it next to the build config, sorry for being unclear 😅

[jbonofre]

Thanks for the details @telemenar and @lriggs .

Can we add a note in the docs folder, quickly documenting this ?

It's a good enhancement to me, we should give a bit of context for the users 😄

[lriggs]

Thanks. I added a few notes.