Geo: Add correlation ID to Geo::RequestService requests

What / Why

Because GitLab Geo is composed of multiple services (at least one primary and one secondary site), relevant logs need to have a common identifier to allow debugging across services.

Proposal

Add correlation ID propagation to Geo requests from secondary to primary sites. When a request originates on a Geo secondary (e.g., git clone via HTTP, web UI requests, API calls), the same correlation ID should be forwarded to the primary site, enabling end-to-end request tracing across Geo sites.

Implementation

Propagate the correlation ID from secondary to primary by:

1. Workhorse: Forward the correlation ID via X-Request-ID header when proxying Geo requests to the primary.
2. Rails: Include the correlation ID in Geo::BaseRequest headers for all Geo::RequestService requests.
3. Documentation: Add setup and troubleshooting instructions for configuring correlation ID propagation across Geo sites (including the -propagateCorrelationID flag requirement on the primary).

Note: The primary site's Workhorse requires the -propagateCorrelationID flag to accept incoming correlation IDs.