Add header or support various authentication method to external routing API

[Chaho12]

The current implementation assumes no authentication is needed between the Gateway and the external routing rules API. However, in real-world deployments, this communication often requires a dedicated authentication mechanism, which may differ from the client-to-gateway authentication.

To support this, the Gateway should allow:

• Injecting headers (e.g., Authorization, API key) into the outbound request (which is different than Feature: Allow External Routing Service to Set HTTP Headers #646)
• Or configuring pluggable authentication modules to support various protocols (Bearer, Basic, mTLS, etc.).