Support config sentry cli save directory

[fishead]

I will run flutter clean first to get a clean environment when I build flutter app, but flutter clean will remove .dart_tool dir, and this plugin store sentry-cli binary in .dart_tool dir, so it has to download sentry-cli every time when I build the app.

Support config sentry-cli save directory to store the binary file in a different place to avoid to download it every time.

[buenaflor]

Thanks for the issue, We'll be looking into this