add 'cluster settings set' settings completion (#67)

pkg/es/cluster_settings.go

@@ -26,6 +26,10 @@ import (
 	"github.com/urfave/cli/v3"
 )
 
+func ClusterSettingsNames(conf *cfg.Config) ([]string, error) {
+	return validClusterSettings, nil
+}
+
 func ClusterSettingsList(conf *cfg.Config) error {
 	res, err := conf.DefaultCluster.ES().Cluster.GetSettings().
 		FlatSettings(true).