Add code signing for Windows app

[FeodorFitsner]

Useful resources:

• https://www.electronforge.io/guides/code-signing/code-signing-windows
• https://melatonin.dev/blog/code-signing-on-windows-with-azure-trusted-signing

[7576457]

I see this as an option for

flet pack <script> --sign-cert <file.pfx> --sign-pass <pass>

where the .exe would be signed using signtool after the build. If these parameters are set in the environment variables, they should be taken from there.

I can take this on.

[FeodorFitsner]

I was mostly looking into Azure code signing as these days signing with your own certificate is challenging: a) certificates come on hardware token which gives a problem with CI and b) if it's not an expensive EV certificate it will have issues with SmartScreen. Even with EV it could give issues with SmartScreen if a cert is not of specific, Microsoft-friendly, issuer. Speaking on my own experience.