Put adding x-gitlab-base-url header behind an FF

What does this MR do and why?

Put adding x-gitlab-base-url header behind an FF

It basically inverts the current feature flag value, but it safe because:

• The feature flag is no-op for self-managed
• The feature flag is globally enabled for .com
• Inverting the value allows us to enable direct-http for devs

Enabling direct-http can give us insights whether a particular problem happens due to how requests are processed via gRPC actions

Related issues:

• Duo Agent Platform connection issues (&19253)
• [FF] `duo_agent_platform_disable_direct_http` -... (#561716 - closed)
• [FF] `duo_agent_platform_enable_direct_http` --... (#569655)