[mlir][polynomial] implement add for polynomial data structure (#92169)

A change extracted from https://github.com/llvm/llvm-project/pull/91655

,
where I'm still trying to get the attributes working for elementwise
constant folding of polynomial ops. This piece is self-contained.

- use CRTP for base classes
- Add unit test

---------

Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>