[16.0][ADD] account_payment_term_installment: add new module

[kaynnan]

@Escodoo [HT01913]

@WesleyOliveira98 @CristianoMafraJunior @marcelsavegnago

This module adds installment generation support to payment terms. It allows:

• Setting a desired number of installments on a payment term
• Configuring the interval in days between each installment
• Automatic generation of payment term lines whenever the installment count or the interval is changed, producing equal percent installments plus a final balance line

Usage:

1. Go to Accounting > Configuration > Payment Terms
2. Open or create a payment term
3. Set the Number of Installments field to the desired value
4. Optionally adjust the Days Between Installments (default 30)
5. The payment term lines are regenerated automatically as equal installments separated by the configured interval, with the last line set as the balance

[WesleyOliveira98]

Functional and Code Review - OK

[kaynnan]

ping @OCA/accounting-maintainers

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[kaynnan]

ping @OCA/accounting-maintainers