[FIX] sale: set and propagate payment type in payment flow

The payment.transaction type field allows filtering some potential
payment methods down the line of a transaction being created; e.g. if a
subscription product is present in a quote, the payment's type should be
`form_save` to save the payment token for future use on the subscription
that will be created at confirmation.

The transaction that got created by sale flows did not set the type of
the transaction correctly, and this value was not propagated to the
rendering values of the acquirer. It now is.

This is necessary as part of task 2275051, where we need to know which
payment methods to enable in Stripe depending on a need for tokenization
or not (since some methods are not tokenizable).

closes odoo/odoo#54426

X-original-commit: 09dbbd77
Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>