Purpose
=======
Currently the wage_type and the schedule pay are defined on the
salary structure type level, preventing users paying different
employees on the same structure with different salary types or
periodicity, which is a employee choice on certain countries,
like Mexico or the USA.
Specification
=============
Move the logic at the contract level, but keep the structure
type field definition to be used as default values.
Part-of: odoo/odoo#124222