Add code signing option to `flet build macos` command

[FeodorFitsner]

Related to #2265 (comment)

Solution: https://stackoverflow.com/a/60025374/1435891

[FeodorFitsner]

I'm not sure it should be a part of flet build command as it doesn't require any special handling from our side.
But we could document that in the docs.

[ndonkoHenri]

• https://marketmix.com/de/codesign-script-flet-desktop-apps-macos/
• https://github.com/hschneider/sign-flet-macos