[IMP] hr_timesheet: Move timesheet_cost into a separate module.

The field timesheet_cost defined on hr_employee in hr_timesheet would be
useful in other modules (eg planning and MRP) as well. Therefore with this
commit the field is moved into a separate module upon which
hr_timesheet will have a dependency. In addition this field is renamed
to employee_hourly_wage, which is a more accurate and neutral name for
this field, as it will not only be used in timesheets.

task-2929518

closes odoo/odoo#97988

Related: odoo/enterprise#30359
Related: odoo/upgrade#3853
Signed-off-by: Kevin Baptiste <kba@odoo.com>