Every repo in the Conjur OSS suite has a license header in every source code file

Add a header to the top of all source code files (e.g. `.go` and `.rb` files, etc) with:
```
// Copyright (c) 2020 CyberArk Software Ltd. All rights reserved.
// Use of this source code is governed by an Apache 2.0
// license that can be found in the LICENSE file.
```