Default payment method per POS device and provider

Commit message

[IMP] point_of_sale, ...: default payment method per provider

..., pos_bancontact_pay

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When several payment methods of a POS config share the same payment
provider (e.g. 5 Adyen terminals), the payment screen displayed all of
them, while a device usually always uses the same one.

Similarly to the default receipt printer, a default payment method can
now be selected per device for each provider. While no default is
selected, a single provider button is displayed in place of its payment
methods; clicking it opens a popup to select the default, then adds the
payment line. Once selected, only the default payment method is
displayed. The defaults can be changed from the "Payment Methods" item
of the burger menu, next to "Printers".

Providers opt out through the new `use_default_payment_method` flag of
the PaymentInterface (true by default).

The payment interfaces setup is moved from the pos store into a new
PosPaymentMethodPlugin, which also handles the default payment methods.

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Task: https://www.odoo.com/odoo/project/1737/tasks/6651478