Do not generate lunch messages on public holidays

[ThorstenBuss]

Problem

Currently, Lunchbot generates messages even on public holidays. This can be disruptive and unnecessary, as many team members may not be working. The fact that the canteen is closed on public holidays can lead to incorrect messages.

Proposed Solution

• Lunchbot should not generate lunch messages on public holidays.
• The list of public holidays should be checked from an API, file, or other data source.
• The configuration should be easy to update or customize for different regions or organizations.

Acceptance Criteria

• Lunchbot skips message generation on any date listed as a public holiday in the configured source.
• The location of public holiday data can be changed without code changes (e.g., via a config file or environment variable).
• Clear documentation on configuring the public holiday source.

Additional Context

This feature will help reduce unnecessary notifications and improve the relevance of Lunchbot's messages.

[ThorstenBuss]

A simpler solution might be to skip the message generation on days where the canteen dos not offer a menu. This probably head the same effect.