Add artifact commands

Implement build, push, pull and tag artifact commands.
For authentication purposes, all `flux <verb> artifact` commands are using the '~/.docker/config.json' config file and the Docker credential helpers.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/pull.go

@@ -0,0 +1,31 @@
+/*
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package main
+
+import (
+	"github.com/spf13/cobra"
+)
+
+var pullCmd = &cobra.Command{
+	Use:   "pull",
+	Short: "Pull artifacts",
+	Long:  "The pull command is used to download OCI artifacts.",
+}
+
+func init() {
+	rootCmd.AddCommand(pullCmd)
+}