[18.0][IMP] account_payment_term_extension: extract payment days logic to a…

[MarinaAForgeFlow]

… new method

Extract the payment days logic to a new method so that the logic can be easily inherited or modified.

[github-actions[bot]]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days. If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[edescalona]

Hi @MarinaAForgeFlow, could you improve the test pass rate? Thanks