Sign and upload policy server binaries in the CI

[jvanz]

Maybe we could sign the binary and upload the binary and its signature to the interim artifacts.
The signatures could be checked when building the container image. I suppose this may be needed for SLSA.
Not much improvement for our current security stance, as we depend on GHA and artifacts anyway. But in the future we could build in our own worker.

Originally posted by @viccuad in #312 (comment)