Currently, when a discount reward is applied onto an order and a tip is added later, the discount gets also applied on the tip.

Steps to reproduce:
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* Make sure all demo data loyalty programs can also apply to the restaurant
* Make sure tipping is enabled in the restaurant
* Open restaurant
* Open a table, add items to the order
* Activate coupon code "10pc"
* Select payment
* On payment page, add a tip (10$)
* Go back to product page
> Observation: 10% discount is also applied on the tip

Why the fix:
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A tipping should never have a discount applied on it.

During the computation of the discountable amount we have 3 distinct flow (discount applies on the order, on the cheapest line, on some specific products)

- On the order: We always exclude tipping lines
- Cheapest line: When fetching the cheapest line amongst all lines we don't consider the tipping lines
- On specific: With the fix, tipping lines will always be excluded from the discountable lines. Even if the tip product was specified on the loyalty program. Should we still allow discounting tips if it was set in the program specifically?

We're using `is_discountable` instead for `is_tip` in case we want to extend the definition later on.

opw-6445364

Forward-Port-Of: odoo/odoo#286073
Forward-Port-Of: odoo/odoo#281400