restic: introduce Counter interface to decouple from ui/progress (#21861)

decouple restic and ui/progress packages

internal/ui/progress/counter_test.go

@@ -58,11 +58,3 @@ func TestCounter(t *testing.T) {
 
 	t.Log("number of calls:", ncalls)
 }
-
-func TestCounterNil(_ *testing.T) {
-	// Shouldn't panic.
-	var c *progress.Counter
-	c.Add(1)
-	c.SetMax(42)
-	c.Done()
-}