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/client.go

@@ -0,0 +1,165 @@
+package cli
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+
+	zotErrors "github.com/anuvu/zot/errors"
+)
+
+var httpClient *http.Client = createHTTPClient() //nolint: gochecknoglobals
+
+const httpTimeout = 5 * time.Second
+
+func createHTTPClient() *http.Client {
+	return &http.Client{
+		Timeout: httpTimeout,
+	}
+}
+
+func makeGETRequest(url, username, password string, resultsPtr interface{}) (http.Header, error) {
+	req, err := http.NewRequest("GET", url, nil)
+
+	if err != nil {
+		return nil, err
+	}
+
+	req.SetBasicAuth(username, password)
+
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		if resp.StatusCode == http.StatusUnauthorized {
+			return nil, zotErrors.ErrUnauthorizedAccess
+		}
+
+		bodyBytes, _ := ioutil.ReadAll(resp.Body)
+
+		return nil, errors.New(string(bodyBytes)) //nolint: goerr113
+	}
+
+	if err := json.NewDecoder(resp.Body).Decode(resultsPtr); err != nil {
+		return nil, err
+	}
+
+	return resp.Header, nil
+}
+
+func isURL(str string) bool {
+	u, err := url.Parse(str)
+	return err == nil && u.Scheme != "" && u.Host != ""
+} // from https://stackoverflow.com/a/55551215
+
+type requestsPool struct {
+	jobs      chan *manifestJob
+	done      chan struct{}
+	waitGroup *sync.WaitGroup
+	outputCh  chan imageListResult
+	context   context.Context
+}
+
+type manifestJob struct {
+	url          string
+	username     string
+	password     string
+	outputFormat string
+	imageName    string
+	tagName      string
+	manifestResp manifestResponse
+}
+
+const rateLimiterBuffer = 5000
+
+func newSmoothRateLimiter(ctx context.Context, wg *sync.WaitGroup, op chan imageListResult) *requestsPool {
+	ch := make(chan *manifestJob, rateLimiterBuffer)
+
+	return &requestsPool{
+		jobs:      ch,
+		done:      make(chan struct{}),
+		waitGroup: wg,
+		outputCh:  op,
+		context:   ctx,
+	}
+}
+
+// block every "rateLimit" time duration.
+const rateLimit = 100 * time.Millisecond
+
+func (p *requestsPool) startRateLimiter() {
+	p.waitGroup.Done()
+
+	throttle := time.NewTicker(rateLimit).C
+
+	for {
+		select {
+		case job := <-p.jobs:
+			go p.doJob(job)
+		case <-p.done:
+			return
+		}
+		<-throttle
+	}
+}
+
+func (p *requestsPool) doJob(job *manifestJob) {
+	defer p.waitGroup.Done()
+
+	header, err := makeGETRequest(job.url, job.username, job.password, &job.manifestResp)
+	if err != nil {
+		if isContextDone(p.context) {
+			return
+		}
+		p.outputCh <- imageListResult{"", err}
+	}
+
+	digest := header.Get("docker-content-digest")
+	digest = strings.TrimPrefix(digest, "sha256:")
+
+	var size uint64
+
+	for _, layer := range job.manifestResp.Layers {
+		size += layer.Size
+	}
+
+	image := &imageStruct{}
+	image.Name = job.imageName
+	image.Tags = []tags{
+		{
+			Name:   job.tagName,
+			Digest: digest,
+			Size:   size,
+		},
+	}
+
+	str, err := image.string(job.outputFormat)
+	if err != nil {
+		if isContextDone(p.context) {
+			return
+		}
+		p.outputCh <- imageListResult{"", err}
+
+		return
+	}
+
+	if isContextDone(p.context) {
+		return
+	}
+
+	p.outputCh <- imageListResult{str, nil}
+}
+
+func (p *requestsPool) submitJob(job *manifestJob) {
+	p.jobs <- job
+}