Problem
-------
Since
https://github.com/odoo/odoo/commit/d3b18979bfb5496fa4370fdf03a806c469127730

,
when a visitor register for an event, the fields in the POST are checked against a list of allowed fields.

The list is hardcoded inside the method _process_attendees_form.

Solution
--------
Move this list in a specific method that can be easily overidden by
another module

closes odoo/odoo#66757

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>