Add signing support on Windows via a certificate authority

[matlu]

Warning message when installing latest version of the app from: https://github.com/project-robius/robrix/releases

[kevinaboos]

hmm, did that happen with prior releases? I haven't encountered this before, but I also didn't change anything about Windows packaging lately.

I'm not sure if there's anything we can do about that, short of publishing it in the Windows store.

[matlu]

I have not tried prior releases.
I think a signed certificate is needed. Publishing in Windows Store should not be needed.
https://stackoverflow.com/questions/74559696/what-are-the-steps-to-certify-my-exe-file-for-external-users

[kevinaboos]

ok, so this requires a paid certificate for use with the Trusted Signing project, so this isn't a simple change. We can apply for a free one via the SignPath Foundation, which I'll do, but it takes time and we may not instantly qualify.