OPSEXP-2656 Add slack file upload composite action

[gionn]

Checklist

• Jira Reference (also in PR title): OPSEXP-2656
• README updated after adding/changing behaviour of an action
• Proposed version increment for release:
  • Patch (bugfix)
  • Minor (new feature)
  • Major (breaking changes)
• External PR link where changes has been tested: license-generator#19

Description

In the past we adopted adrey/slack-file-upload-action but it's currently unmaintained:

• Looks deserted adrey/slack-file-upload-action#32

There is now a pletora of low stars forks, all made as javascript action which are hard to inspect from the security point of view, e.g.:

• https://github.com/aabed/slack-file-upload-action
• https://github.com/kibettheophilus/slack-file-upload
• https://github.com/yanskun/slack-file-upload-action
• https://github.com/vaporif/slack-file-upload-action

Hence I prefer we maintain a simple composite action using the upstream slack python sdk to do the job.

[pmacius]

Looks good. Is it tested somewhere?