Introduce "polling_interval_multiplier" as application setting

What does this MR do?

Adds "polling_interval_multiplier" to application settings. Implements module for setting "Poll-Interval" response header. Returns 429 in ETag caching middleware when polling is disabled.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29759 (closed)

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1539