POS: Caisse-AP payment protocol for France

This module adds support for the **Caisse AP** protocol over IP in the
Odoo Point of Sale.

The [Caisse AP
protocol](https://associationdupaiement.fr/protocoles/protocole-caisse/)
is a vendor-independent protocol used in France to communicate between a
point of sale and a payment terminal. It is implemented by
[Ingenico](https://ingenico.com/fr/produits-et-services/terminaux-de-paiement)
payment terminals, [Verifone](https://www.verifone.com/) payment
terminal and other brands of payment terminals. This protocol is
designed by a French association called [Association du
paiement](https://associationdupaiement.fr/), abbreviated as **AP**.
Note that the Caisse-AP protocol is used by Ingenico payment terminals
deployed in France, but not by the same model of Ingenico payment
terminals deployed in other countries!

This module support a bi-directionnal link with the payment terminal:

1.  it sends the amount to the payment terminal
2.  it waits for the end of the payment transaction
3.  it parses the answer of the payment terminal which gives the payment
    status: in case of success, the payment line is automatically
    validated ; in case of failure, an error message is displayed and
    the Odoo user can retry or delete the payment line.

The Caisse-AP protocol was initially written for serial and USB. Since
the Caisse AP protocol version 3.x, it also supports IP. When used over
IP, the client (point of sale) and the server (payment terminal)
exchange simple text data encoded as ASCII over a raw TCP socket.

The Caisse-AP protocol has one important drawback: as it uses a raw TCP
socket, it cannot be used from pure JS code. So the JS code of the point
of sale cannot generate the query to send the amount to the payment
terminal by itself. In this module, the JS code of the point of sale
sends a query to the Odoo server that opens a raw TCP socket to the
payment terminal. It implies that, if the Odoo server is not on the LAN
but somewhere on the Internet and the payment terminal has a private IP
on the LAN, you will need to setup a TCP port forwarding rule on the
firewall to redirect the TCP connection of the Odoo server to the
payment terminal.