Add portage support for Gentoo Linux (#1076)

Co-authored-by: Christopher Phillips <christopher.phillips@anchore.com>

internal/formats/syftjson/model/package.go

@@ -157,6 +157,12 @@ func unpackMetadata(p *Package, unpacker packageMetadataUnpacker) error {
 			return err
 		}
 		p.Metadata = payload
+	case pkg.PortageMetadataType:
+		var payload pkg.PortageMetadata
+		if err := json.Unmarshal(unpacker.Metadata, &payload); err != nil {
+			return err
+		}
+		p.Metadata = payload
 	default:
 		log.Warnf("unknown package metadata type=%q for packageID=%q", p.MetadataType, p.ID)
 	}