Activity indicator for init command (#207)

* first version of spinner

- implemented class with basic method
- covered with dummy test
- integrated with init command

* Style and license remarks

* fixed review remarks

* fixed typo + integration of spinner with terminate command

* integration of spinner with create command

cli/internal/cmd/spinner.go

@@ -0,0 +1,73 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+package cmd
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/spf13/cobra"
+)
+
+var (
+	spinnerStates = []string{"⣾", "⣽", "⣻", "⢿", "⡿", "⣟", "⣯", "⣷"}
+	dotsStates    = []string{".", "..", "..."}
+)
+
+type spinner struct {
+	out      *cobra.Command
+	text     string
+	showDots bool
+	delay    time.Duration
+	wg       *sync.WaitGroup
+	stop     int32
+}
+
+func newSpinner(c *cobra.Command, text string, showDots bool) *spinner {
+	return &spinner{
+		out:      c,
+		text:     text,
+		showDots: showDots,
+		wg:       &sync.WaitGroup{},
+		delay:    100 * time.Millisecond,
+		stop:     0,
+	}
+}
+
+func (s *spinner) Start() {
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+
+		for i := 0; ; i = (i + 1) % len(spinnerStates) {
+			if atomic.LoadInt32(&s.stop) != 0 {
+				break
+			}
+			dotsState := ""
+			if s.showDots {
+				dotsState = dotsStates[i%len(dotsStates)]
+			}
+			state := fmt.Sprintf("\r%s %s%s", spinnerStates[i], s.text, dotsState)
+			s.out.Print(state)
+			time.Sleep(s.delay)
+		}
+
+		dotsState := ""
+		if s.showDots {
+			dotsState = dotsStates[len(dotsStates)-1]
+		}
+		finalState := fmt.Sprintf("\r%s%s  ", s.text, dotsState)
+		s.out.Println(finalState)
+	}()
+}
+
+func (s *spinner) Stop() {
+	atomic.StoreInt32(&s.stop, 1)
+	s.wg.Wait()
+}