nested struct types in record batch API

[yogendra-17]

Pull Request description

Adds struct columns to the record batch API (nested type; the list half landed in #101).
A struct column holds N named fixed-width primitive fields. Child types are now stored as a ColDesc{type, name, children} tree instead of a flat per-column vector, so list-of-struct / struct-of-list can reuse the same representation later.
Stacked on feat/arrow-list-types (#101); retarget to main once that merges.

How to test these changes

• Build the native runtime, then pytest packages/irx/tests/test_record_batch.py -q.

Pull Request checklists

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• the tests generate log file(s) (path).
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• The code is well commented, especially in the parts that contain more
  complexity.
• New and old tests passed locally.

Additional information

Depends on #101

Reviewer's checklist

Copy and paste this template for your review's note:

## Reviewer's Checklist

- [ ] I managed to reproduce the problem locally from the `main` branch
- [ ] I managed to test the new changes locally
- [ ] I confirm that the issues mentioned were fixed/resolved .

[yuvimittal]

@yogendra-17 , thanks for working on this
can you rebase this PR
One correctness gap: get_struct only checks is_null for the whole struct slot, then reads each field's fixed-width buffer directly (child[row]) without consulting the field's own null bitmap. IRx-built data can't hit this, but a PyArrow-imported struct like {"x": 10, "y": None} would return a garbage/zero y instead of None. Worth either honoring the child validity bitmap or documenting that field-level nulls are unsupported in this cut .

but we can work on this later on

[yuvimittal]

i tried rebasing your branch, but there are some merge conflicts so can you please look into this

[yogendra-17]

@yuvimittal I've resolve the merge conflicts,can you please help me review and merge?