Add HTTP authentication support

[michelseiler]

Summary

This Pull Request introduces a new HTTP-based authentication variant designed to provide a simple and platform-independent way to extend authentication with an external application.

Motivation

Existing authentication solutions did not work reliably in my setup. I initially attempted to use plugin_authn, but this feature is not supported when running the official Docker Alpine image. I also tried ext_auth, but this approach failed under load, frequently hitting OS-level limits when handling a large number of requests.

Proposed Solution

The newly added HTTP authentication variant offers a lightweight and flexible mechanism for integrating external authentication logic. It avoids the limitations encountered with prior methods and provides a clean, portable, and easy-to-implement alternative that works consistently across environments.

Key Benefits

• Platform independent: Works without relying on features unavailable in Alpine-based images.
• Easy integration: External applications can reliably perform authentication by exposing a simple HTTP server interface.
• Robust under load: Avoids OS constraints encountered with previous approaches.
• Straightforward implementation: Keeps configuration and extension effort minimal.