PURPOSE

Before this commit, the "payment from odoo" flow (direct) required
tokenizing a payment method before processing a payment, rather than
directly processing the payment, and eventually tokenizing the payment
method later. In practice, this often meant that a 'validation'
transaction had to be processed and immediately refunded in order to
generate a temporary token. Then, a second "payment by token" flow had
to be executed to process the intended payment.

This implementation was too rigid for most modern payment providers'
APIs. Indeed, they mostly expect single processing for a given payment
and usually offer the possibility to tokenize the payment method after
the payment, rarely before. Because of this, the implementation of many
providers was either difficult, limited, or even impossible.
Among the various incompatibilities, we find: support for direct
payments but not tokenization, lack of support for authentication during
tokenization, absence of hosted page dedicated to tokenization, etc.
As another cascading consequence of this implementation choice, several
providers could not be migrated to newer APIs after that the ones
implemented in Odoo were deprecated.

The goal here is thus to 'invert' the payment flow implemented in Odoo.
As this means re-writing most of the payment module and of its provider
implementations, the opportunity is taken to deeply clean and document
the code of the impacted modules.

SPECIFICATIONS

- Lift the limitations listed above by inverting the generic direct
  payment flow to first create and process a transaction, then tokenize
  it if requested.
- Remove the `payment_flow` field on `payment.acquirer` and let the
  acquirer choose the appropriate flow according to the use-case.
- Filter acquirers offered to the customer based on the use-case.
- Add overridable hooks at key steps of the payment flow to allow
  implementing new providers with minimal effort (both in Python and
  JavaScript).
- Move all module-specific logic and fields to where they belong (e.g.
  let Subscriptions filter out acquirers based on their support for
  tokenization, move `qr_code` in `payment_transfer`, ...).
- Homogenize the inheritance strategy of acquirer modules.
- Standardize the implementation logic in other modules' controllers.
- Improve the traceability of payments through stored fields and logs.
- Remove the public read right on `payment.acquirer` and enforce the
  use of access tokens in all modules' flows.
- Fix all bugs that were inherent to the old implementation.
- Replace the previous testing suite (which was mostly commented-out for
  years) with a new one that allows testing of both routes and flows
  with different test configurations (user, transaction context, ...).

LINKS

Enterprise PR: https://github.com/odoo/enterprise/pull/12528
Upgrade PR: https://github.com/odoo/upgrade/pull/2291

task-2085989

Co-authored-by: Antoine Vandevenne <anv@odoo.com>
Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Arnaud Joset <arj@odoo.com>
Co-authored-by: Kevin Baptiste <kba@odoo.com>
Co-authored-by: Barad Mahendrasinh <mba@odoo.com>
Co-authored-by: Prakash Prajapati <ppr@odoo.com>
Co-authored-by: Adrien Horgnies <aho@odoo.com>