Sign builds with GnuPG

[TheAssassin]

Successful build can be found here: https://github.com/AppImage/type2-runtime/actions/runs/8953447788/job/24591970512

I intend to share the private key with @probonopd. Please get in touch via either e-mail or chat.

Fixes #24.

[probonopd]

Let's document what needs to be done in X years when the signature expires.

[TheAssassin]

I'd like to add the key to the files that are uploaded to the releases. Should we ever change it, that will make communicating the change easier (at least I hope so).

[TheAssassin]

what needs to be done in X years when the signature expires

Well, that depends. There are many options. One could change the expiry date and thus extend the key. Alternatively, one could generate a new key. Both options have their pros and cons, and we don't know what the future will bring.