Deduplicate command execution and logging code in Kanister functions

[e-sumin]

Describe the bug

We have multiple locations where we execute commands within a pod and log the command output:

[pkg/function/backup_data_stats.go]
		var stdout, stderr bytes.Buffer
		err = commandExecutor.Exec(ctx, cmd, nil, &stdout, &stderr)
		format.LogWithCtx(ctx, pod.Name, pod.Spec.Containers[0].Name, stdout.String())
		format.LogWithCtx(ctx, pod.Name, pod.Spec.Containers[0].Name, stderr.String())

It would be nice to refactor these places and reuse duplicated code.

[ShivangShandilya]

I would like to work on this issue !!