Adding support for personal snippet endpoint on the API

What does this MR do?

Exposing the snippets feature as an API

Are there points in the code the reviewer needs to double check?

• it's a first shot, and I'm not very comfy with Ruby/RoR, so all of it! (fixed)
• the create method has no authentication yet, and when uncommented does not work (403). (fixed)
  • But is it really needed? I mean can the API really be used when having no private key? (fixed)
• The /snippets/public (or /snippets/all or /snippets/explore) cannot be reached… I guess something's wrong. (fixed)
• I have no idea where to start to do a /snippets/search endpoint, but that would be useful. (fixed)
• there's no tests yet. (done)
• there's no doc yet. (done)

Why was this MR needed?

Because the personal snippet feature had no API implementation…

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#22042 (closed)