Add dart3 test with for hybrid horizontal scaling with load balancer

[iapicca]

Description

This PR introduces a new test implementation that reworks the horizontal scaling approach attempted in #10493 by leveraging a reverse proxy/load balancer.

Motivation

Native Dart horizontal scaling is currently limited by dart-lang/sdk#62545, which prevents effective inter-process port sharing.

To bypass this limitation, this implementation utilizes Traefik to orchestrate and load-balance multiple Dart worker instances running on unique ports.

As this approach significantly deviates from the default Dart implementation, I believe it serves as a better standalone test to demonstrate the true performance capabilities of Dart once the current scaling bottlenecks are removed.

The default Dart 3 implementation is being separately restored in #10632, upon which this PR is based.

Changes

• Added the dart_hybrid test, including a custom multi-stage Dockerfile.
• Added Traefik configuration files and a run.sh script to manage the hybrid process lifecycle.
• Implemented dynamic port parsing via command-line arguments in the Dart server.