Add endpoint to add public-holiday task to every employee

[x-m-el]

This needs the backend change that every employee has their own graph for their timesheet: redpencilio/app-timekeeper#4

Adds an endpoint /populate-public-holidays to add a "public holiday task" worklog to every employee.

Possible extension could be to allow passing the type of task in another endpoint, giving more options for worklog population.

[erikap]

@x-m-el Can you add a mu-script such that a public holiday can be added via CLI. E.g. mu script admin-jobs add-public-holiday 2024-01-01. Support for 1 date at a time is sufficient.

Something similar is done in the kimai-sync service: redpencilio/timekeeper-kimai-sync-service@a583925