Add License headers to all the files under the x-pack directory

[ph]

We would like to make it explicit which files is cover under what license, right now everything that lives under the x-pack folder is under the Elastic License. All modules in the x-pack/files/modules.d are under the Elastic License by default.

But at the moment the following files *.JSON, *.YML in the x-pack folder does not declare a license.

We need to make sure that:

• Manifests
• Pipelines
• Dashboards

Are all covered by the Elastic headers.

Things we need:

• We need to check the presence of the license headers for theses files when running make check
• Having tooling in place to automatically add headers to the files.

[ph]

List of unique file type in the directory ordered by the number of files., Some of them are binary and we cannot add any headers like the JPG.

    358 json
    330 yml
     88 dat
     69 asciidoc
     46 log
     21 md
     15 tmpl
     13 py
     12 plain
     11 png
      7 Makefile
      7 Dockerfile
      6 js
      6 gitignore
      5 evtx
      5 csv
      3 yaml
      3 txt
      3 j2
      3 bat
      2 pcap
      2 jpg
      1 wtmp
      1 svg
      1 status
      1 rl
      1 rb
      1 placeholder
      1 examples
      1 Corefile
      1 btmp_ubuntu1804

[botelastic[bot]]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!