🦋 Allow shop owner to customize delivery fees VAT

[Tirodem]

Allow shop owner to choose either for delivery fees specifically :

• VAT null (because he doesn't pay VAT at Poste with his status)
• VAT = VAT from shop country
• VAT = custom VAT rate

[coyotte508]

note that to be consistent for DB storage (& cart display), we'll need to associate a country even with custom vat rate

[Tirodem]

I don't have any objection for that, as long as the setted rate is applied 👌