[15.0][IMP] sale_invoice_plan: add select type down payment of invoice plan

[Saran440]

This PR implement module sale_invoice_plan for user can select type of down payment

Normally, when user use invoice plan and has advance. it will use down payment with type percentage only.

Example Case

• User create sale order 1 line with product A 100us and there is tax 7%
• Use invoice plan and advance 10%
• When create invoice by plan. Down payment will compute by percent and get value = 10.7 (include tax)

Some case user need down payment exclude tax. So, I add new field advance_type for user selected down payment with Fixed amount or percent

[OCA-git-bot]

Hi @kittiu,
some modules you are maintaining are being modified, check this out!

[Saran440]

@rousseldenis Fixed. Add test script for this PR. THank you for your review