SSH signatures support

[fpoussin]

Hi,

SSH signed commits are currently ignored because the field type was unknown.

This PR adds basic support for it.

[palantirtech]

Thanks for your interest in palantir/policy-bot, @fpoussin! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks! This looks good for the has_valid_signatures and has_valid_signatures_by predicates. As a follow-up, we should probably add a key_fingerprints property to the has_valid_signatures_by_keys predicate, but I think this is fine to merge for partial SSH support.

[bluekeyes]

Hey @fpoussin, it turns out this is incompatible with GitHub Enterprise 3.6 and earlier, which do not support SSH signature. Running the GraphQL query fails with the error: No such type SshSignature, so it can't be a fragment condition

I'm going to see if there's a way to enable the feature only when the target GitHub instance supports it, but I might have to temporarily revert this PR until GitHub Enterprise 3.7 is more widely deployed (probably early in 2023.)