Add allowed_download_modes object storage configuration

What does this MR do?

Exposes allowed_download_modes in gitlab.rb and renders it into gitlab.yml for consolidated and existing storage-specific object storage settings.

Related Rails change: gitlab!252350 (merged) Companion chart MR: gitlab-org/charts/gitlab!5312 (closed)

Related issues

Related to gitlab#624081 (closed)

Checklist

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review.
• The UBT version and corresponding checksum hash have been updated and referenced in the merge request if applicable.
• The UBT EE pipeline is green.

Expected

• Test plan: Ruby syntax, ERB compilation, editor diagnostics, YAML parsing, and git diff --check pass. Focused RSpec tests could not run because Bundler is unavailable.
• Documentation created/updated. The gitlab.rb configuration template is updated.
• Tests added for consolidated and existing storage-specific object storage.
• Integration tests added to GitLab QA. This change serializes configuration only.
• Equivalent MR for the GitLab Chart opened: gitlab-org/charts/gitlab!5312 (closed).
• The setting renders as JSON and accepts Rails-supported proxy and direct array values.