Unified Backups: Load SourceContext from configuration data

What does this MR do and why?

This Merge Request decouple the following entries in SourceContext from the main rails codebase:

• gitlab_version
• backup_basedir
• ci_builds_path
• ci_job_artifact_path
• ci_secure_files_path
• ci_lfs_path
• packages_path
• pages_path
• registry_path
• terraform_state_path
• upload_path
• env

Context

The gitlab.yml configuration files includes a lot of deprecated (yet still supported) ways to configure things. Some of the defaults (when missing) or some of the fallbacks are implemented via config/initializers/1_settings.rb file.

Here is an example of the Builds Artifacts: https://gitlab.com/gitlab-org/gitlab/-/blob/454530-unified-backups-load-rails-context-from-data/config/initializers/1_settings.rb#L295-304

Settings.artifacts['storage_path'] = Settings.absolute(Settings.artifacts.values_at('path', 'storage_path').compact.first || File.join(Settings.shared['path'], "artifacts"))
# Settings.artifact['path'] is deprecated, use `storage_path` instead

It claims to prefer storage_path but it will use path if present, and still have the shared/artifacts fallback.

While in the case of CI secure_files we have storage_path as the preferred value, a fallback to shared folder without a path fallback:

Settings.ci_secure_files['storage_path'] = Settings.absolute(Settings.ci_secure_files['storage_path'] || File.join(Settings.shared['path'], "ci_secure_files"))

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #454530 (closed)