Add project support to unsigned_commit_shas for SSH commit signatures

The following discussion from !214739 (merged) should be addressed:

• @vyaklushin started a discussion:

  issue: I found this code that only supports commit shas without projects.

      # Find commits that are lacking a signature in the database at present
      def unsigned_commit_shas(commit_shas)
        return [] if commit_shas.empty?
  
        signed = by_commit_sha(commit_shas).pluck(:commit_sha)
        commit_shas - signed
      end
  

  source

  It's used in BranchHooksService to create commit signatures after the git push.

  signature_types
    .map { |signature| signature.unsigned_commit_shas(commit_shas) }
    .reduce(&:&)

  source

  It shouldn't be a problem right now, since CreateCommitSignatureWorker doesn't support Ssh::Commits.

      # Instantiate commits first to lazily load the signatures
      commits.map! do |commit|
        case commit.signature_type
        when :PGP
          Gitlab::Gpg::Commit.new(commit)
        when :X509
          Gitlab::X509::Commit.new(commit)
        end
      end

  source

  But it sounds like a follow up issue. WDYT? 🤔