cli: add config and images command

Extends the existing zot CLI to add commands for listing all images and
their details on a zot server.
Listing all images introduces the need for configurations.

Each configuration has a name and URL at the least. Check 'zot config
-h' for more details.

The user can specify the URL of zot server explicitly while running the
command or configure a URL and pass it directly.

Adding a configuration:
zot config add aci-zot <zot-url>

Run 'zot config --help' for more.

Listing all images:
zot images --url <zot-url>

Pass a config instead of the url:
zot images <config-name>

Filter the list of images by image name:
zot images <config-name> --name <image-name>

Run 'zot images --help' for all details

- Stores configurations in '$HOME/.zot' file

Add CLI README

pkg/cli/image_cmd.go

@@ -0,0 +1,123 @@
+package cli
+
+import (
+	"strconv"
+	"time"
+
+	zotErrors "github.com/anuvu/zot/errors"
+	"github.com/briandowns/spinner"
+	"github.com/spf13/cobra"
+)
+
+func NewImageCommand(searchService ImageSearchService, configPath string) *cobra.Command {
+	searchImageParams := make(map[string]*string)
+
+	var servURL string
+
+	var user string
+
+	var outputFormat string
+
+	var imageCmd = &cobra.Command{
+		Use:   "images [config-name]",
+		Short: "List hosted images",
+		Long:  `List images hosted on zot`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if servURL == "" {
+				if len(args) > 0 {
+					urlFromConfig, err := getConfigValue(configPath, args[0], "url")
+					if err != nil {
+						cmd.SilenceUsage = true
+						return err
+					}
+					if urlFromConfig == "" {
+						return zotErrors.ErrNoURLProvided
+					}
+					servURL = urlFromConfig
+				} else {
+					return zotErrors.ErrNoURLProvided
+				}
+			}
+
+			var isSpinner bool
+
+			if len(args) > 0 {
+				var err error
+				isSpinner, err = isSpinnerEnabled(configPath, args[0])
+				if err != nil {
+					cmd.SilenceUsage = true
+					return err
+				}
+			} else {
+				isSpinner = true
+			}
+
+			err := searchImage(cmd, searchImageParams, searchService, &servURL, &user, &outputFormat, isSpinner)
+
+			if err != nil {
+				cmd.SilenceUsage = true
+				return err
+			}
+
+			return nil
+		},
+	}
+
+	setupCmdFlags(imageCmd, searchImageParams, &servURL, &user, &outputFormat)
+	imageCmd.SetUsageTemplate(imageCmd.UsageTemplate() + usageFooter)
+
+	return imageCmd
+}
+
+func isSpinnerEnabled(configPath, configName string) (bool, error) {
+	spinnerConfig, err := getConfigValue(configPath, configName, "showspinner")
+	if err != nil {
+		return false, err
+	}
+
+	if spinnerConfig == "" {
+		return true, nil // spinner is enabled by default
+	}
+
+	isSpinner, err := strconv.ParseBool(spinnerConfig)
+	if err != nil {
+		return false, err
+	}
+
+	return isSpinner, nil
+}
+
+func setupCmdFlags(imageCmd *cobra.Command, searchImageParams map[string]*string, servURL, user, outputFormat *string) {
+	searchImageParams["imageName"] = imageCmd.Flags().StringP("name", "n", "", "List image details by name")
+
+	imageCmd.Flags().StringVar(servURL, "url", "", "Specify zot server URL if config-name is not mentioned")
+	imageCmd.Flags().StringVarP(user, "user", "u", "", `User Credentials of zot server in "username:password" format`)
+	imageCmd.Flags().StringVarP(outputFormat, "output", "o", "", "Specify output format [text/json/yaml]")
+}
+
+func searchImage(cmd *cobra.Command, params map[string]*string,
+	service ImageSearchService, servURL, user, outputFormat *string, isSpinner bool) error {
+	spin := spinner.New(spinner.CharSets[39], spinnerDuration, spinner.WithWriter(cmd.ErrOrStderr()))
+	spin.Prefix = "Searching... "
+
+	for _, searcher := range getSearchers() {
+		found, err := searcher.search(params, service, servURL, user, outputFormat,
+			cmd.OutOrStdout(), spinnerState{spin, isSpinner})
+		if found {
+			if err != nil {
+				return err
+			}
+
+			return nil
+		}
+	}
+
+	return zotErrors.ErrInvalidFlagsCombination
+}
+
+const (
+	spinnerDuration = 150 * time.Millisecond
+	usageFooter     = `
+Run 'zot config -h' for details on [config-name] argument
+`
+)