WIP: Save full-trace as artifacts (FileStorage only)

What does this MR do?

Concept

1. Runner picks a job
2. Runner sends chunks of the trace. Unicorn appends the chunks to Gitlab::Ci::Trace#default_path
3. Runner sends the full-trace. Unicorn saves to JobArtifactUploader#default_path, and remove live-trace in Gitlab::Ci::Trace#default_path.

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

/cc @ayufan @nolith