[IMP] hr_work_entry_contract: prep work for multiple sources

Prep work to allow work entries to be created from multiple sources when
using contracts.
The end goal is to be able to generate work entries from attendance as
well as the planning app.
After that the plan is to make sure the implementation works well with
payroll for hourly wages.

This commit contains the necessary changes for the next work_entry
modules to work.

TaskId-2466871