Fix authentication parameters for Git streaming audit events SSH requests

Summary

The log_git_streaming_audit_events feature flag rollout revealed authentication issues when GitLab Shell makes requests to the new /shellhorse/git_audit_event endpoint. The current implementation only supports username-based authentication, but GitLab Shell has requests from different actors, i.e., keyless SSH CERT mode operations.

Background

• Feature flag log_git_streaming_audit_events was globally enabled but had to be reverted due to high volumes of 404 errors.
• The new Git audit event workflow uses a dedicated /shellhorse/git_audit_event endpoint instead of the previous /allowed endpoint.
• Current authorization is narrowed to usernames only, causing failures for other authentication methods.

Expected Behavior

GitLab Shell should pass all necessary authentication parameters to support different actor types when making requests to the Git audit event endpoint.

Current Behavior

GitLab Shell requests fail with 404/401 errors for non-username authentication methods.