ci: attest build provenance for images

[blocktrron]

This PR adds GitHubs functionality of attesting proof a artifact object is generated from the GitHub actions CI and it's respective commit.

My goal with the usage of GitHub Actions for the ffda firmware was initially to ensure transparency of how and from which references the images are built. I see this functionality as another great step towards ensuring transparency (even though nobody will probably ever use it).

This is a example run: https://github.com/blocktrron/site-ffda/actions/runs/9636954817

More Info:
https://docs.github.com/en/actions/security-guides/using-artifact-attestations-to-establish-provenance-for-builds
https://github.blog/2024-05-02-introducing-artifact-attestations-now-in-public-beta/

[blocktrron]

Also i think it makes sense (according to the docs) to attest the manifests.

We can also do this at a later stage, as I'm unsure on which conditions this should be done and how to reflect it in the build pipeline.

So let's tackle this step by step.