Support native crash reporting for Windows

[bruno-garcia]

Sentry SDK for Flutter supports Windows since it's preview versions: #407
Since then, Flutter has made Windows support GA: https://medium.com/flutter/announcing-flutter-for-windows-6979d0d01fed

But it does not capture a native crash. The goal is to use sentry-native with the crashpad backend to capture minidumps on Windows.

This was recently done on the Unity SDK:

• Windows native support sentry-unity#527

This tracking issue contains the items we would consider when adding support to Flutter. Including:

• Offline caching on by default
• Crash free rate to rely on the native layer (ask native: did I crash last run?)
• Scope sync so data set to the SDK in Dart, shows up in native crashes
• Make sure we can bundle crashpad_handler.exe with the final app
• Upload symbols automatically during build. (might require changes to: https://github.com/getsentry/sentry-dart-plugin)

Related:

• Support split-debug-info for Flutter Desktop (Linux and Windows) #433
• Support offline caching (Dart, Windows, Linux) #487
• Document ffiPlugin pubspec.yaml key, and its implications on existing plugins that use FFI flutter/flutter#108974
• crash can not be captured on windows sentry-native#818

[marandaneto]

Consider this issue when tackling this.