Add /internal/geo_proxy endpoint [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds an internal endpoint for Workhorse to use to know that it should act as a Geo secondary site and proxy most requests to the primary.

Introduces feature flag geo_secondary_proxy which is disabled by default. When disabled, this endpoint returns empty data.

In FOSS, this endpoint returns empty data.

Resolves #324664 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not user facing, it is an internal API endpoint.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.