add option to download VAT-Spy data automatically

[dneuge]

Instead of manually releasing proxy updates and/or requiring users to link manually downloaded VAT-Spy files, the proxy should receive an option to automatically download the latest released version as announced by VATSIM.

The proxy has been designed to be as minimal as possible, so this option was intentionally left out as it requires a permanent folder to be specified for caching the retrieved data. Such an option starts to make more sense now that further functional updates to the proxy should be quite rare, so essentially most releases will only happen to update the included data. Regular updates to the proxy should still happen but by providing users an option to update VAT-Spy data automatically, the intervals could be increased to twice per year or whenever an update needs to be done for other reasons than just updating VAT-Spy data.

The option must not be enabled by default.

[dneuge]

Information about released version and download URLs is available from https://api.vatsim.net/api/map_data/

[dneuge]

The file would need to be checked if it can be parsed (plus maybe a small plausibility check) before replacing the previous files, otherwise issues like https://github.com/vatsimnetwork/vatspy-data-project/issues/674 would break previously working data (the typo led to the file not being able to be opened at all).