Ensure ButlerSDK.Core tool ButlerTool_RestAPI_GetPublicHolidays gets more verify input and data flow.

[ShadowKnightMK4]

The just of the matter is we currently blindly trust the external data source: "https://date.nager.at/api/v2/publicholidays" <- which very nice

Not that the plan is to assume bad but a few points:

1. what if the local DNS is curropted and https://date.nager.at/api/v2/publicholidays for the PC is currently pointed to an attacker controlled server?
2. ensure the request can be made without blocking input
3. Ensure the result is actually a list of holiday sin a properfly formatted json.
4. Enforce a timeout on the Http request.;