Add formula for CA AGI additions variable

The `ca_agi_additions.py` module does not have a formula.

It would seem that additions should include (at a minimum) `alimony_income` given the [Schedule CA instructions](https://www.ftb.ca.gov/forms/2021/2021-540-booklet.pdf#page=47) for Line 2a - Alimony Received.

Start with the `ca_agi_additions.py` module and review the linked Schedule CA instructions for Line 2a, Alimony Received. Confirm which additions are required, including `alimony_income`, and define the formula so the module no longer lacks one. Done means the formula reflects the documented minimum additions.

Written by the indexing model from the issue text.