WIP: Implement the implied CI/CD config for AutoDevOps

What does this MR do?

Create a fallback strategy for when the GitLab CI/CD config is missing. Behind an application setting, so disabled by default.

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

Includes parts of gitlab-org/gitlab-ce#34777