Add offline caching support for desktop

[tustanivsky]

Sentry supports offline caching of the events on both Android and Apple platforms. However, this functionality is currently lacking for desktop platforms, specifically on Windows and Linux.

sentry-native library used by the Unreal Engine plugin on desktop discards envelopes if they are scheduled to send while the network is down. The only scenario where an event is resent is when the server rate-limits the request.