[Feature]: mTLS support for upstreamconnections

Please select the area the issue is related to

Gateway

Please select the aspect the issue is related to

Aspect/API (API backends, definitions, contracts, interfaces, OpenAPI)

Suggested Feature

@subject

Related Issues

No response

Steps to Verify

• Design Document — A detailed design document has been created and reviewed, covering architecture, data flow, and edge cases.
• Design Mail — A design summary email has been sent to relevant stakeholders for awareness and feedback.
• Code Review — All code changes have been peer-reviewed and approved according to the project's review standards.
• Testing Complete — Adequate unit, integration, and/or end-to-end tests have been written and are passing.
• Documentation Review — User-facing and/or developer documentation has been updated to reflect the new feature and reviewed.
• Feature Complete — The feature is fully implemented, all checklist items above are done, and it is ready for release.

The issue does not identify source files, tests, entry points, or a concrete behavior beyond adding mTLS support for upstream connections. Begin by locating the gateway's upstream connection and TLS configuration code, then define the design and tests needed to demonstrate secure mutual-TLS connections before implementation.