[IMP] website_sale: add hook to _get_checkout_steps for easier override

`_get_checkout_steps` didn't provide a friendly way to add steps in the
checkout flow. Developers had to reimplement part of the parent logic in 
their overrides.

This commit introduces `get_checkout_step_list`. This method will allow
an easier override of the checkout steps while keeping the logic to 
return a single step in the parent method.

closes odoo/odoo#158026

Signed-off-by: Valentin Chevalier <vcr@odoo.com>

addons/website_sale/models/website.py

@@ -564,14 +564,9 @@ class Website(models.Model):
 
         return self.is_view_active('website_sale.address_b2b')
 
-    def _get_checkout_steps(self, current_step=None):
+    def _get_checkout_step_list(self):
         """ Return an ordered list of steps according to the current template rendered.
 
-        If `current_step` is provided, returns only the corresponding step.
-
-        Note: self.ensure_one()
-
-        :param str current_step: The xmlid of the current step, defaults to None.
         :rtype: list
         :return: A list with the following structure:
             [
@@ -620,6 +615,19 @@ class Website(models.Model):
             'back_button':  _lt("Back to cart"),
             'back_button_href': '/shop/cart',
         }))
+        return steps
+
+    def _get_checkout_steps(self, current_step=None):
+        """ Return an ordered list of steps according to the current template rendered.
+        If `current_step` is provided, returns only the corresponding step.
+        Note: self.ensure_one()
+        :param str current_step: The xmlid of the current step, defaults to None.
+        :rtype: list
+        :return: A list containing the steps generated by :meth:`_get_checkout_step_list`.
+        """
+        self.ensure_one()
+
+        steps = self._get_checkout_step_list()
 
         if current_step:
             return next(step for step in steps if current_step in step[0])[1]