[IMP] hr_holidays: separate value preparation and record creation into 2

methods for others to extend functionality

1. _prepare_resource_leave_vals_list() that will prepare value for
creation
later in _create_resource_leave()
2. _create_resource_leave() now calls the
_prepare_resource_leave_vals_list()
instead of having done the job itself

closes odoo/odoo#84775

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>