feat(poc): holiday assignment

[rtdany10]

This is a PR with bare minimum functionality of holiday list assignment, like shift assignment.
An extra parameter as_on is introduced in the existing function with None as default.
The existing function is also made to call the new function with the help of a hook.

Related PR on erpnext: frappe/erpnext#45803

I'll work on the validations and rest of the clean up if this approach seems acceptable to the maintainers.

[rtdany10]

@asmitahase @AyshaHakeem @ruchamahabal @nabinhait
Sorry to tag, but a review would be nice

[rtdany10]

Hi @rtdany10, I'm not clear as to what problem does this solve. A little more context would be helpful!

Hi @asmitahase,
Currently, holiday list is not "assigned", rather set in employee/shift/company.
Assume you have a holiday list: 2025, starting from 01/01/2025 and ending on 31/12/2025.

I assign this holiday list inside employee. Everything works fine till end of the year. 2026 starts and a similar new Holiday list is created and set inside the employee on 01/01/2026.

Now, lets say I'm running the payroll for December 2025 by 5th of Jan, 2026. System will show all holidays as absent for the employee as their holiday list has been updated to 2026. To tackle this, I'd like to propose a Holiday Assignment, much like Shift Assignment.

[asmitahase]

Thanks @rtdany10, we'll get this reviewed.