Only consider signatures from emails listed in the gemspec during signature verification

[rochlefebvre]

The GemVerifier class currently deems a gem signed if there is at least one "valid" signature in Rekor, i.e. one with a valid cert chain and signature matching the gem file's digest. It doesn't care who signed it, except that it separates emails listed in the gemspec vs. the ones that aren't.

Given how rekor accepts any signatures for a given artifact, we need to be more discerning w.r.t. the signatures that have some authority.

In the prototype, we should ignore non-maintainer signatures by default, and perhaps only print them if requested (but they're kinda worthless imo).